microsoft has two ways of doing this
you call them or go online and they ask a series of questions and tell you it needs repair. then they ask:
method#1
MS sends you a box with all the foam packaging, a pice of tape and a return sticker. you put your xbox in and drop it off or call ups for pick up. (this is what I do, the down side is you must first wait on a box then send it off, adds like a week to repair time before it even leaves the house)
method #2
they will email you a return mailing lable, you put it in your box, put the lable on it, drop it off or call for pick up. (the down side is you dont have the protecting foam and packaging, you must buy your own box. the good is it saves about a week of extra shipping.
let me know if I can jhelp anymore.
when sending make sure NOT to send anything but the xbox, no harddrive, no memory card, controller, custom face plate. nothing send it bare, chances are you wont get the same xbox back, even if you do, they are not responcable for these items and the are discarded during disasembly.