1. I'll get to it. -- I'll attend to it or reach it (at some unspecified future time). Often implies that there are other things to be done first, so whatever it is may have to wait.
2. I'll get on it. -- I'll attend to it (usually promptly).
3. I'm on it. -- I'm attending to it now, or will start immediately.
4. I'll get it over with. -- It's not something that I w