Don't use it. You'll either sound like a "wannabe tough guy," or like someone who understands English poorly. Most native speakers don't even bother with using such a sloppy phrase. It's a mumbled way of saying "I'm going to", if you really weren't sure of what it should mean.

"Imma" is the informal way to say "I am" or "I'm". It is generally used in text messages between friends and is considered silly, but is still widely used amid American youths and occasionally adults. :)