"Ain't" is considered a nonstandard contraction. But it is also part of some people's natural dialect.
Sticking to formal forms is better for writing. And, I wouldn't say "ain't" around anyone you want to impress, since it is considered by many to be a contraction that only uneducated people use.
"Ain't" is a contraction that can replace the negative contractions in any of the following phrases. So the affirmative would be the contractions without "not" in them.
Aren't I? Ain't I?
Am I?
I'm not... I ain't...
I'm..
Aren't you? Ain't you?
Are you?
You aren't. You ain't.
You're...
Isn't he, she, it? Ain't he, she, it?
Is he, she, it?
He, she, it isn't... He, she, it ain't...
He's, she's, it's
Aren't we? Ain't we?
Are we?
We aren't.... We ain't...
We're...
Aren't they? Ain't they?
They aren't. They ain't...
Are they?
They're...