Connect EA account to Xbox 360 account

Download the NCAA 13 Demo and when you start the game it will ask you for a email. Enter the email address you want and that email will be link to your gamer tag at least as far as ea is concerned.

if its not linked yet (for example when you never before played an EA game with the gamertag) you can link it over Origin.
Log into Origin with your EA/Origin account, try to add a friend and on the left side you have an option to link facebook/PS3/Xbox360 accounts