I'd add that you shouldn't use it when writing academic or other official stuff.

Hi Thierry! Generally, ain't is a contraction of am not/are not/is not/have not/has not. Like isn't and aren't or haven't. So instead of "he isn't crazy" one might say "he ain't crazy". It's an American/regional thing though, so it might come across as a bit weird if used by random foreigners (like me). Cheers, Tim

Ain't isn't technically an official word, but slang. It isn't a true word as there is no "A not" the way there is for don't (Do not) or Isn't (Is not). It is very common in the South where I live. Another one to avoid is Y'all, which is a slang abbreviation for You all that is also best left unsaid. If you do want to learn to use ain't, use it in a manner in place of isn't, aren't, am not, etc.. He ain't helping fix my tire You ain't from California, are you? I ain't reading that.

very informal, probably best not to learn it