http://www.rewards1.com/index.php?referrer_id=737695

That code brings you to a web site. Notice that you can actually get lots of things for free, not just free Xbox codes (but the free xbox codes are the easiest to get).

Click "Sign Up" on the left, create an account and verify it with your email address. There's no spam so stop worrying.

Now you're in! Congratulations, you're ready to start claiming your free Xbox codes. (less info)