“Ain’t” is deprecated, that is, it is not accepted in the standard language (although you'll hear it a lot in music). Especially in American English, “ain’t” is considered to be “uneducated”, and learners should not even consider using “ain’t” unless and until they find that all their native speaker friends use it. Originally, “ain’t” was the contraction of “am not”, but nowadays in non-standard English “ain’t” can be used as: am not, is not, are not, and sometimes has not and have not. Note: since standard English has no contraction for “am not”, you’ll often hear “aren’t I” used in questions, especially tag questions. This is considered normal these days, although you may want to say “am I not” in formal situations. Personally, I always say “am I not”, but that’s a bit idiosyncratic :)

Yes, like Phil says, don't use "ain't" unless native speakers are using it around you AND you want to mimic them. Very rare to hear it (if ever) by native UK English adult speakers. And I never heard it any more when I was in Canada where English is more a mix of UK English and NA English). When you say it many people will not like it. It doesn't sound good and people may think you "uneducated". If you have a strong native English accent, they may put it down to where you come from and not care so much. But a non-native speaker using it, they may not be as forgiving.

The difference is education ... A fuller answer : think of it as poetry, useful for songwriters for rhyme and scansion, but not really for borrowing yourself. The "full story", of course, is more complex. It''s a feature of English native speaker cultures, I think, that there is often proud resistance to the "dominant elite", expressed through many kinds of accents, dialects and behaviour in opposition to what used to be the old ruling classes. Many people using "ain't" and many other non-standard usages and idioms are in fact highly educated, and fully aware that "The Man", the dominant culture, doesn't like it. The choice of language is part of a declaration of cultural identity - I am with the people, and not with the aristocracy, the grammar Nazis, the ruling classes, the power elite. Still, it's another word to know about, but probably to avoid using too much yourself until you are sure you know what you are doing :)