You'll need an Action Replay, found at most electronic retailers, or you'll need to learn how to read, identify, and change coding for the game in order to do that.
There are no ingame cheats for Halo 2, just button glitches.
[Edited by yosup, 7/4/2008 12:28:03 PM]
Button glitches are like meleeing some one and shooting at the same time (R & B)