General Theory

Fit for a purpose

Generally, the more tasks you want a fit to do the worse it performs at each particular task. Some ships have very specific uses: a Venture mining frigate is not very helpful in either PvE or PvP combat.

But even a combat ship will almost certainly need different fits for PvE and PvP, because, for example, in PvP you usually receive a lot more damage in a much more limited time than in PvE, and enemy players, unlike NPCs, need to be prevented from warping away. Even within PvP the same ship can have one fit for close-range brawling in a small gang and a completely different fit for fighting at longer ranges in a large fleet.

Of course, there are dangers in over-specialisation too, especially when you're not working with other players. If you're going to exploit a low-class wormhole while solo you want your ship to deal and tank damage, launch probes, and maybe cloak so that you can hide if you see a gang of enemies on your directional scanner.

According to Jester guide for PvP fitting, one should consider the following elements (along with a rough guide to their range):

Consider your ship's bonuses

Every ship comes with per-level bonuses, which often point towards the uses that the ship was designed for.

The Caldari Blackbird, for example, gets a 15% bonus to ECM Target Jammer strength and a 10% bonus to ECM Target Jammer optimal range for each level its pilot has in the Caldari Cruiser skill. These bonuses suggest that it's best fit as an electronic warfare platform which fights well at long range.

Sometimes it's okay not to take advantage of a ship's bonuses: although the Brutix is bonused to give a more efficient active armor tank, it is almost always seen with a buffer tank, and quite often a shield tank at that. This is because the ship has the ability to supply a huge amount of DPS with its six damage bonused blasters and a shield tank leaves the lowslots free for damage upgrades to squeeze out even more damage.

Don't mix tanks

If you're going to tank (and not all fits include a tank) either shield tank or armor tank. Don't shield tank and armor tank in the same fit.

Most of the time a tank takes up a substantial proportion of your powergrid, your CPU and either your midslots (for a shield tank) or your lowslots (armor tank). If you fit a shield tank, you can put useful things (damage modules, speed and agility modules &c) in your lowslots, and if you armor tank you can put useful things (tackling modules, electronic warfare modules, propulsion modules &c) in your midslots.

If you mount two tanks, even if they're both buffer tanks, your ship won't be very useful except as a target.

Don't mix guns

If you have a rack of exactly identical guns on your ship, they will all have precisely the same optimal and falloff ranges, and exactly the same tracking. This means you only have to worry about getting your ship to one ideal range and keeping the enemy's angular velocity below one value.

Furthermore, you can group identical weapons so that they can all be activated or reloaded or unloaded at once, which reduces the micromanagement required in combat. Grouping isn't always the best option, especially if you learn to overheat your guns using the Thermodynamics skill, but it's a good way to start. (There are, however, cases where having mixed guns due to fitting reasons can squeeze a bit more DPS from your ship.)

Missiles are a little different, because they aren't affected by tracking and have a simpler kind of range, but the same principle often applies to them. Note that 'don't mix guns' doesn't mean you shouldn't put missile launchers in extra highslots if all your turret hardpoints are full and you want more DPS (as in, for example, a Rifter with three autocannons and a missile launcher) -- though there are other options for spare highslots, such as energy neutralisers/vampires, salvagers, tractor beams, remote repair modules and drone link augmentors.

Bigger guns are not necessarily better

Bigger guns do more DPS, assuming they hit, but they track targets more slowly and they use up more powergrid and CPU.

So, for example, there are three kinds of medium-sized autocannon (the short-ranged kind of projectile weapon): Dual 180mm, 220mm, and 425mm. The 425mm autocannons do a bit more damage, but the 220mm autocannons have much more forgiving fitting requirements (especially if your fitting skills -- see below -- need more training). On some ships it may be that fitting a rack of 425s would use up so much powergrid and CPU that you wouldn't be able to fit a tank. In that situation, fitting 220s would be better because, even though you would do a little less damage per second, your ship would survive longer and so apply more total damage.

(This does not mean that you should fit frigate-sized guns on a cruiser. Downsizing within the available medium or large guns is sometimes wise, but downsizing from large to medium guns, or from mediums to smalls, usually isn't, unless you're fitting a bait ship or certain kinds of drone boat fits.)

Try fits outside the game

There are a number of programs which let you try out out a fit hypothetically, outside the game.

The most popular is the EVE Fitting Tool, which you can acquire here. EFT can tell you about the performance and fitting requirements of a particular fit. If you import your character's skills into it it can give you numbers accurate for your character. This lets you check that you can fit everything you want on to a particular ship before you buy the ship and modules.

EFT's numbers aren't always perfectly accurate, and it's possible to be caught up in the numbers so that you forget the practical realities of piloting in EVE -- EFT can import your character's skills but it can't factor in your own piloting skills and experience. Nevertheless, it's a very useful tool.

If you can't use EFT (e.g. Mac user) or just want an alternative, try Pyfa. It offers all the functionality of EFT, including DPS graphs (however Pyfa's are less powerful). It is written in Python and is open source.

Steal other people's ideas

EVE University has two ship loadout forums, where pilots can get comments on their fitting ideas. They have some threads with suggested fits. It's also worth searching the forums for past threads if you're interested in a particular ship.

The PvE Ship Loadouts and PvP Ship Loadouts forums should be the first place new players look for fits, and ask for advice. This forum is restricted to EVE University members and alumni.

The wiki also has a page for each ship in eve (eg. Atron). These pages often have recommended fits and skills to help you get an idea for how to fit the ship, and are a good starting place to get a sense of how the ship flies, although there is no alternative for flying a couple for yourself.

You can also get advice in the University's chat channels. (To link a fit that you have set up already into chat, drag the ship's name from the fitting window and drop it into the chat entry box; to link one from the fittings browsing window, drag the image of the ship.)

If you want to go further afield there's a forum for EVE players called Failheap Challenge which has fairly comprehensive pvp and pve ship fittings forums. Bear in mind that fittings suggested on Scrapheap are usually aimed at players with a lot of skillpoints, and a lot of the PvP fittings are for small gang or solo PvP. It's also a forum with a fairly aggressive and competitive posting culture which doesn't suffer fools, new members or people who break the rules (or any combination of the three), gladly.

The subreddit devoted to EVE ship fitting, r/fittings has active discussions of ship fittings and posts asking for suggested fits for specific play styles and ships.

There is also evefitfinder.com, which collects fittings from zkillboard.com and by this gives you examples of what people fit their ships with. Take this with a grain of salt however, not everybody fits their ship smartly.

Fitting modules (don't overuse them)

There are some low-slot modules and rigs which increase your powergrid or CPU, and so are sometimes called 'fitting modules'. See below for how to decide on which fitting module to use.

It is sometimes necessary to use a fitting module or a fitting rig, but if you have a fit that requires more than one it's often a bad sign, and it may mean that you need better fitting skills.

Train fitting skills

Fitting skills reduce the CPU or powergrid requirements of modules, or just give you more raw CPU or powergrid to play with. Having decent fitting skills is very useful, and having very good fitting skills really helps to fit T2 modules and weapons, which demand more CPU and powergrid than their T1 equivalents. Levels in these skills are often required to fit useful modules, too. The fitting skills are:

Mining Upgrades 5% less CPU need for mining upgrade modules (useful for miners but, of course, only for miners)

(Note that Hull Upgrades doesn't make basic fitting easier, even though it has 'Upgrades' in its name. It gives you 5% more armor per level and Hull Upgrades at Lvl 5 is required to fit a Tech 2 armor tank.)

Tip: The benefits of CPU Management and Power Grid Management apply to the whole ship. Skill point for skill point, they provide significantly more fitting benefit than the module specific skills. You can't go wrong training these two skills to level 5 as soon as practical.

Choosing the Modules (PvP)

(Content from this section is shamelessly inspired from Jester's guide, see the Further reading section.)

Once you have selected the the engagement range, role, and tank of your ship, comes the time to actually fit it. Here is a guide on how to actually do this. Note that this guide is PvP orientated. While it may have some applications for PvE fits be cautious following this guide for them.

Weapons

For ships where damage is the primary purpose, start with weapons that are consistent with your engagement range and ship class: laser beams for snipers, artillery for skirmishers, blasters or rockets for brawlers etc. Try to use the most damaging weapons of the class that you've selected. You may have to downgrade them later (or may choose to downgrade them later so that everything will fit, or to improve tracking), but for now, if you're fitting blasters, try to fit neutron blasters, and so on.

Do not mix weapons and stick to weapons that go with your ship's bonuses. In addition, fit as many weapons as your ship has either missile or gun hard-points. Finally, do not mix weapons of the same type but of different "grades". For instance, do not mix 180mm and 220mm autocannons on the same ship.

In a similar fashion, if the ship is designed to act as logi, the first thing to do is to fit the remote repair modules. If you are fitting an EWAR ship, start by fitting the EWAR modules that match your ship bonuses.

Role Enhancing Modules

Now look at your ship's role. If your role is damage, you're probably already in good shape but start to consider tracking. If you are going to be shield tanking, by default you should be using two damage-increasing modules consistent with your weapons in the lows. You might end up with one, you might end up with three. But start with two. If you have more than four low slots and will be using guns, you'll probably want to fit at least one Tracking Enhancer module as well. If you are going to be armor tanking, by default you should have one damage-increasing module consistent with your weapons in the lows.

This also applies to ECM ships: if you are shield-tanking, fit two Signal Distortion Amplifiers. If you are armor-tanking, try to find room for one. This is generally a good rule of thumb for enhancing the damage or the other effects that your ship puts out.

Finally, armor-tanking gun-ships with lots of mid-slots should consider a Tracking Computer in one or perhaps two of them.

Nearly all PvP ships should give a single mid slot over to a propulsion module, something to increase your ship's speed. You must choose between a Microwarpdrive (MWD) or Afterburner (AB). A MWD allows greater manoeuvrability which makes them more commonly fit. However ABs are popular on brawling armor ships as they enhance their natural signature tank and cannot be shut off by warp scramblers (often found at close ranges).

Most PvP ships should give a single mid slot to a tackle module of some type. For skirmishers (specialised scouts), primary damage, and screen ships this should nearly always be a long point, a Warp Disruptor module of some kind. Ships in a tackle role should usually fit a Warp Scrambler module of some kind. Ships in other roles can forgo tackle modules.

ECM is a large danger to logistics so you should consider fitting ECCM to counter it. A sensor booster (either to counter sensor dampening or to allow your to apply reps faster) should also be considered.

Tank

Next, consider your tank. First, fit a Damage Control unit of some kind. T2 should always be favoured however meta 3 or 4 can be used due to dramatically less CPU usage (bear in mind that meta 4 damage controls are quite costly).

Buffer armor tanks use between two (generally for T1 frigates) and six (generally for battleships or logistics) low slots. In order, fit the following:

the heaviest armor plate your ship can fit consistent with its size and your guns (i.e. 200 or 400mm for frigates and destroyers, 800 or 1600mm for cruiers, 1600mm for battlecruisers and battleships). You should fit T2 if you can, downgrading to meta 4 otherwise.

An Energized Adaptive Nano Membrane (EANM), T2 if you can

A second T2 EANM, or an Adaptive Nano Plating (ANP) if it won't fit

NB: A T2 ANP is both easier to fit and gives better bonuses than a meta EANM

look at your four resistances and "close" the one that is lowest with a single active armor hardener of the appropriate type

Remember that on DD ships you want to leave room for at least one damage module (eg. magnetic field stabiliser for hybrids), and often two.

Buffer shield tanks operate in a very similar fashion but with one fewer slot, using between two and five. In order, fit the following:

a Large Shield Extender (for cruisers and larger) or a Medium Shield Extender (for frigates and destroyers). Small Shield Extenders should never be used, on any ship

one Adaptive Invulnerability Field, or if cap is an issue (and you are only using two slots) possibly a second shield extender of the same type

look at your four resistances and "close" the one that is lowest with a single active shield hardener of the appropriate type

a second Adaptive Invulnerability Field

if you have the power grid for it, a second Large Shield Extender

Active armor tanks operate in a similar fashion, but generally replace the plates with Armor Repair Modules plus one Ancillary Armor Repair module of the appropriate size. They may also replace EANMs with a second (and sometimes even third!) armor repairer. Active shield tanks replace the Shield Extenders with one or perhaps two Shield Boosters, the first of which is usually an X-Large Ancillary Shield Booster (cruisers and up) or Medium Ancillary Shield Booster (frigates). Use caution if intending to fit a Large Ancillary Shield Booster. In most applications, it will not repair sufficient damage to be useful. The Small Ancillary Shield Booster should not be used on any ship.

Passive shield tanking is not normally used for PvP.

Fitting mods

At this point, you are possibly running out of power grid, CPU, or both. It is at this point that you begin to have to consider using "fitting mods." Fitting mods operate by closing the gaps in your fit to allow everything you want to use to fit on the ship. In general, you should try not to use fitting mods unless the fit absolutely demands it. The six most common fitting mods are:

Ancillary Current Router rig

Power Diagnostic System low slot module

Reactor Control Unit low slot module

Micro Auxiliary Power Core (MAPC) low slot module.

Co-Processor low slot module

Processor Overclocking Unit rig

The first four increase your power grid; the MAPC is for greatly increasing grid on frigates - and is almost mandatory on every shield tanked frigate - and the RCU is for greatly increasing grid on ships bigger than frigates. The last two increase your CPU. In general, try to use the rig first before sacrificing a valuable low-slot module. Under almost no circumstances is fitting more than two fitting mods a good choice. If you find yourself fitting more than two fitting mods, you have probably made an incorrect choice in your ship fit somewhere. Fit meta 4 shield extenders or plates; reduce the number of shield extenders or plates; or reduce the guns you have chosen by one grade, from (for instance) neutron blasters to ion blasters, or from 425mm autocannons to 220mm autocannons.

As you adjust the fit of your ship, do not be afraid to change some of the modules from T2 to meta modules, usually meta 4 modules. This most often applies to Shield Extenders, your Damage Control, your point or scram, and your afterburner if not already meta (microwarpdives should never be T2). These changes will give you back a couple of percent of power grid or CPU here and there and are often all that's needed to bring a fit into line if the fit is close. It is usually not a good idea to change your resistance modules for meta modules; they are far less effective than T2 modules.

Speed Tanking

Speed-tanking operates in a similar fashion, but reduces the maximum number of tanking modules available to one or two, usually focusing on shield. Most often, a Medium Shield Extender (often of the meta variety) and a Damage Control module are the two modules chosen. Alternately, other ships work well with two Large Shield Extenders and a Damage Control. In the general case, if you have only a few tanking modules on a ship, it is better to increase buffer with those that you do use unless you are also flying with a logistics ship, in which case you can replace one with a single module to increase your resists.

Final Slots

At this point, you may have a low slot or two free. You may have a mid slot or two free. And you may have a high slot or two free. High slots are the easiest, and the only ones you should consider leaving empty (due to high fitting requirements of the options). For ships in a tackle role, a NOS is usually the best choice. For ships in all other roles, a neut is usually the best choice. A free low slot should be given to an additional damage module, an additional Tracking Enhancer, or (if you're running short of CPU), a Nanofiber Internal Structure. An additional mid slot should be given to an additional tackle mod (usually a Stasis Webifier), a Capacitor Booster, or some form of utility electronic warfare, usually a Sensor Dampener.

Inertia Stabilizers should not be fit on PvP ships under any circumstances. Sensor Boosters can be fit on PvP ships but should be fit only with a great deal of care and consideration - the most common being for gate camping. In a gang, this module will nearly always get you made fun of unless it is specifically required for sniping applications or the like.

Finally, modules that passively regenerate some aspect of your ship's operation should almost never be used in PvP. These include Cap Rechargers, Cap Power Relays, Shield Rechargers, and Shield Power Relays. While these modules are fine for PvE, the incoming damage or impacts to capacitor in PvP will generally be too strong for these modules to have much if any effect.

Rigs

Finally and last, rig your ship using any remaining rig slots. In general at the basic level, passive shield tanking ships should use a full set of Core Defense Field Extender rigs. Passive armor ships should use a full set of Trimark Armor Pumps. This will increase the size of your ship's buffer and extend your life on the battle field. Active tanking ships use more specialized rigs. Active armor-tanking ships will use two Auxiliary Nano Pump rigs and one Nanobot Accelerator rig. Active shield-tanking ships will give one or two rig slots over to increasing shield resistances but may also use a Core Defense Operational Solidifier or (much more rarely) a Core Defense Capacitor Safeguard. Of course, if you have given over some rig slots to fitting rigs, you will have fewer rigs to devote to defense.

As with passive regeneration modules, do not use the Core Defense Field Purger rig in PvP at the basic level. While there are advanced level ships that can (and do) use this rig successfully in PvP, at the basic level you should stick with increasing buffer.