Q:

What kind of JOIN should I use here?

I'm new to JOINS in MySql.
I have six tables: t1, t2, t3, t4, t5, t6.
And I also have one main table: main_table.
TOTAL 7 TABLES!
The first column of ALL tables is called classified_id.
If the user searches for "cars" then the main table will match everything in table t1 (which is the cars table) where classified_id is the same in both tables.
So:
   SELECT * FROM main_table, t1 WHERE main_table.classified_id=t1.classified_id

This works fine, although I am not sure this is the way to join here. Performance is an issue in my case!
However, here is my problem.
Whenever ALL CLASSIFIEDS are searched, then I need to match the main_table.classified_id to the other tables classified_id column and get every classified there is. 
How should this query be made up?
    SELECT * FROM main_table, t1, t2, t3, t4, t5, t6 // I have this so far which is not much!

If you need more input just ask and I will update this Q.
Thanks
EDIT:
Table setup:
     main_table:              t1:
     ID(PK)                   ID (PK)
     classified_id -> 25      classified_id     ->   25
     category  -> CARS        year     ->    1997

A:

If the row would exist in all tables (i.e. every table has a row for a specific classified_id), then you would use an inner join:
SELECT 
   m.classified_id
   ,m.category
   ,t1.year
   ,........
FROM 
   main_table m
   INNER JOIN t1 ON m.classified_id = t1.classified_id
   INNER JOIN t2 ON m.classified_id = t2.classified_id
   INNER JOIN t3 ON m.classified_id = t3.classified_id
   INNER JOIN t4 ON m.classified_id = t4.classified_id
   INNER JOIN t5 ON m.classified_id = t5.classified_id
   INNER JOIN t6 ON m.classified_id = t6.classified_id

If the row does not exist in every table, then you'd use LEFT JOINS so that rows are not dropped

