The word "ain't is used as a contraction for are not, is not, has not, and have not. Example: I ain't got no money= I haven't got any money. Ain't you a pretty woman? Aren't you a pretty woman? It ain't so= It isn't so. He ain't here= He is not /isn't here

What 'register' you use depends on who you are talking to. You wouldn't use 'ain't' if you were talking to the Queen or at an interview for a job. If you were having fun with your friends it would be acceptable.

yeah peaches is right but the actual meaning of ain't is"am not"

Jura's answer is good, but you should know that it is not considered proper English. At least not in the U.S. I don't know about UK, Australia, etc. I would not recommend using it. It is very commonly used but it's use is looked down upon as indicating a poor education. Sometimes people use it just for dramatic effect or to be funny.