First, you need to create an account with MSN. Then, after confirming your email, go to the official website of Xbox and register. When doing this, choose a suitable user name and enter you MSN registration address. From here you can upgrade your status, wait for a conformation email, and then wait for a PM in the forums. Once you acquire it, validate it by clicking on it. For more information, visit http://www.wikihow.com/Create-an-Xbox-Live-Gamertag.