Instructions
Start your Xbox 360 without a disk in its tray, and sign into your Xbox 360 account. Select "My Xbox" and choose "System Settings."
Click "Memory."
Highlight "Hard Drive." Press "Y" on your Xbox 360 controller.
Press "Format."
Select "Yes" when prompted to erase your Xbox 360 hard drive. Enter your Xbox 360 serial number if asked (look on the back of your Xbox 360, or behind the oval door on the front of your Xbox 360, in order to find its serial number).