Ain’t is a slang expression, used in spoken language, but not in written language (formal).
• He ain’t very rich. = He isn’t very rich.
• She ain’t got a clue. = She hasn’t got a clue.
• He ain’t heavy, he’s my brother. (Famous song)
You can listen to the podcast from My Happy English. Michael explains everything about Ain’t ain’t a word in the podcast.
Ain’t is acceptable in spoken or when you want to convey a vernacular tone. Omit ain’t in business, scholarly, or any other formal writings unless it is used in direct quotation, and never go around saying it in general conversation unless it is a part of joke or well-known saying. Everyone should use it occasionally as part of a joking phrase, but if you always use it instead of the most proper contractions, you are sure to be branded as uneducated. Treat it like spicy mustard: don’t make a whole sandwich from it.