Turn on the Xbox 360
Press the Guide button on the controller
Go to Settings
Select System Settings
Select Console Settings
Select System Info
Note the console serial number if needed
Go back to System Settings
Select Storage
Highlight the hard drive or storage device
Press the Y button
Select Format
Confirm the format
Enter the console serial number if prompted
Wait for the format to complete
Restart the console
Sign in and set up the Xbox 360 again