The English language has no future tense. To refer to the future, we use various strategies with verbs in present tense (some of them auxiliaries):
I will run
I will be running
I shall run
I’m going to run
I am to run next
I’m running tomorrow
I run next Friday
Because we can conceptualise the future and it plays a big role in our lives, we talk about it often. Naturally, then, the ways we talk about it are subject to pressures of economy, resulting in contraction, e.g.:
I will run → I’ll run
I am going to run → I’m gonna/gon’ run
I’m gonna run → I’mna run → I’ma/Imma run
I’ll is acceptable in Standard English; gonna/gon’ and I’mna/I’ma/Imma are not, though you may see them in dialogue or informal writing or use some of them yourself in everyday speech – gonna is especially widespread.
Recently I came across another form: I’m on [verb]. It seems similar to I’ma and I’m gon’, but I don’t know exactly how or when it developed. Here’s the example I saw, in Elmore Leonard’s novel Mr. Paradise:
“You know who put the stuff on you?”
“Somebody close to me, his girlfriend’s punk-ass brother. Is how it goes. But listen, I’m on tell you something, I was scared.”
“I would be too,” Delsa said.
I’m on [verb] doesn’t appear to be common, at least in written English, though Google led me to this line from Kathryn Stockett’s The Help: “Today I’m on tell you bout a man from outer space.” And in GloWbE I found: “Law have mercy. I reckon I’m on do it.” (from ‘Entrepreneurs are a first world Phenomenon’ by John Egan).
Based on the few examples I’ve seen, my guess is that I’m on [verb], like I’ma, is originally and still chiefly AAVE. But I’m open to correction, and to other thoughts you might have on it. I’m on wait and see now.
I forgot that Mark Liberman looked at this on Language Log a couple of years ago: ‘Gonna, gone, onna, a — on?‘. He begins with a different example from Elmore Leonard (“I’m on get you to the hospital”, from Raylan), and links to an older post, ‘”on” time’, that deals with the same passage I quote above.
Both posts offer helpful analysis of the construction and its various pronunciations and spellings. Thanks to @f_moncomble for the reminder.