Tom: What's up?
Joe: I'm afraid that I'm going to fail my big English test tomorrow because I haven't
really studied much for it.
Tom: Tell you what, dude. I'll come over to your place at 8 p.m., and I'll
help you prepare for the test. As you know, my English is pretty good because I spent
three years in the States.
Joe: You're on, man. ( = I happily accept what you have just suggested.)