I think the others covered it pretty well. One way of thinking of it: AIN'T EVER = use of AIN'T is the only mistake AIN'T NEVER = use of AIN'T and DOUBLE NEGATIVE: two mistakes. However, if you are trying to use a pariticular type of slang/colloquial speech, AIN'T NEVER is probably more natural. The idea is that if you are going to use AIN'T, you're probably going to use DOUBLE NEGATIVES also.

Ain't is colloquial and considered poor, broken English, but in contextual meaning, they are both correct.

In standard English, neither is correct. "Ain't" isn't used in standard English. But in the dialects where "ain't" is used, double negatives are also commonly used. So, you would be more likely to hear "ain't never" than "ain't ever." But it's important to remember that this is not standard English, so you wouldn't want to speak this way in a business or academic environment.

Jonathan is correct. I will just add that in any new language, there are often words and phrases that you don't need to use but you should learn to understand. For example, there is a lot of Mexican slang that would sound ridiculous if I said it, but I need to understand it when I hear it. Good luck!