I dont think this means "i am a..." i have a lot of examples of using it. here is the one from a song.
And if you ask me too Daddy's gonna buy you a mockingbird I'mma give you the world I'mma buy a diamond ring for you I'mma sing for you I'll do anything for you to see you smile And if that mockingbird don't sing and that ring don't shine I'mma
I'm more familiar with it meaning simply "I am" but as the others have said, it can also mean "I am going to." Since it is obviously not standard English, you shouldn't worry too much about it. While you might hear it, you should never see it written down except as dialogue or song lyrics, etc.
In your example, it simply means "I'm going home."
In older English it was more common to place a- before an -ing form (often reduced to -in'). This pattern is retained in some non-standard varieties of present-day English -- to some extent in rural America, for example -- though the pattern has nearly died out, I think, by now.
They were a-dancin' and a-singin' till all hours of the night.