My Lord
Super Freak
Well, you can upload the videos to your Bungie.net account and people can download them and play them through Halo 3 (it just saves the data for the game and doesn't actually make a video file).
If you want to make a video of something to post on the internet (like Youtube) then you'd have to get a capture card for your computer, where you would plug in your Xbox output into the computer and then record it from there. But that would cost money, and if you want a good one that can do HD then it would cost like $100
Thanks Viper, I'm going try through Bungie's site.