How do I play halo 3 without going on xbox live?

we just got an xbox 360 and halo 3 and I dont get how to play it at all. how do I simply play a halo 3 (not on xbox live) just to practice to get better? (oh yeah the reason I dont want to go on xbox live is cause my brother CANNOT find out.)

all you have to do is play the campaign you dont have to go on xbox live. this way you play the actual game you dont have to play against any other people you just be playing the story. I've already beat the game so any other questions just ask me.

I heard you can get a x-box compatiable demo from Walmart...I'm not sure if it's true though.

I have Halo 3 though. =]

Either campain or split screen against a friend...