Instructions
Turn on your Xbox 360 console and access the Dashboard by either removing any discs in the tray or pressing the center "Xbox" button on your controller and pressing "Y."
Log in to the profile you wish to upgrade by selecting it on the main Dashboard screen. Scroll to "My Xbox" and select your avatar by pressing the "A" button.
Press "A" to choose "Join Xbox Live."
Select "Sign Me Up" to begin the registration process.
Follow the on-screen instructions to complete your Xbox Live registration. You will be able to keep your Gamertag if it has not been taken, and your profile points will transfer with the upgrade.