Q:

Dealing with new rules as a new Player

So I'm a beginner to D&D.  I like the character creation and stuff.  My DM keeps changing around things and I'm getting a bit confused. 
Example: Advantage/Disadvantage is now +5 and -5 respectively instead of rolling twice  
I am now told that I cannot dual-wield rapiers of all things purely because they said so.  I'm a newer Player so maybe I am in the wrong here.  

A:

It sounds like your DM is new to 5e as well.  

I'll start with dual wielding.  (Two Weapon Fighting)  
You cannot dual wield weapons unless they are both light weapons.  If you look at the weapon table on page 46 of the basic rules, you will see that the rapier does not have the light property.  

Light. A light weapon is small and easy to handle, making it ideal for
  use when fighting with two weapons. See the rules for two-weapon
  fighting in chapter 9. (Baic Rules, p. 46)  

The rule on Two Weapon Fighting is available to any character of any class. 

Two-Weapon Fighting
  When you take the Attack action and attack with a light melee weapon that you’re holding in one hand, you can use a bonus action to attack with a different light melee weapon that you’re holding in the other hand. You don’t add your ability modifier to the damage of the bonus attack, unless that modifier is negative. If either weapon has the thrown property, you can throw the weapon, instead of making a melee attack with it. (Basic Rules, p. 74)     

If your DM allows feats (they are an optional rule) you can take the feat dual wielder, which in addition to increasing your AC by 1 while dual wielding also allows you to dual wield non-light weapons such as longswords and rapiers.  
This differs from other editions of D&D which merely penalized dual wielding.  
Then the next rule regarding advantage/disadvantage,
Advantage and Disadvantage are essentially a +/-5 bonus/penalty if you need an average roll (it is larger/smaller as you need to hit a lower/higher DC). Is this an optional rule to save rolling time?  Not sure to be honest.
I personally roll two d20s and the damage die when I have advantage, as written in the rules.    

When you have either advantage or disadvantage, you roll a second d20 when you make the roll. Use the higher of the two rolls if you have advantage, and use the lower roll if you have disadvantage. For example, if you have disadvantage and roll a 17 and a 5, you use the 5. If you instead have advantage and roll those numbers, you use the 17.  More detailed rules for advantage and disadvantage are presented in chapter 7. (Basic Rules, p. 4)  

