No matter the arguments to the contrary,
ain't is a word (has been since the 1770s) and it is in the dictionary. Its usage is considered non-standard in every dictionary I checked, but it is used in the everyday speech of most people. I personally never use it, but everyone else in my family does. "Resistance is futile," as the Borg would have us believe.
You need to be familiar with colloquial speech, as you'll be exposed to it quite frequently on most streets in America. There are ways to write colloquial speech, as well, but you would never expect to see it in formal writing. We do speak the language differently than the way it is formally taught - just like Spanish, in spoken form, doesn't quite match the written form.
Nuff's been said 'bout how we speak, but I think song lyrics push the envelope just a tad.