Is ain't supposed to written as ain't?

It is not a proper Word, but traditionally when it is used, it is a contraction for am not. Contractions use an apostrophe.

Example:

I am not going into that dark cave.

I ain't going into that dark cave.

In writing, it should only be used as a slang. For instance, it would help add traits to a character in a novel. However, it should never be used in formal or business writing.