I'mma = I am a It's colloquial english that is typed out... No one types like this, but some people do talk like this in everyday speech

This is a terribly squished-up word. You can stretch it out to "I am going to" (= I'm gonna = I'mma), so "I'mma talk to him" means "I am going to talk to him". Here's an example from STAN by Eminem: "If I have a daughter, guess what I'mma call her? I'mma name her Bonnie." Another variant is " 'monna " (again, "I'm gonna")

Sounds like African American English to me. I'mma = I'm going to...