It is not 'beginners' English, and it isn't something I was familiar with before your question, although I guessed it meant something to do with "I am...".
Just to say that "Imma" is likely to be theinformal abbreviation in speech of "I am to", which is used to show that somehting will hapen in the future. So, in that lyrics, when they sing "Imma be", it is "I am to be" --> future action.