When you start up your Xbox, after picking your location and network settings you will be prompted to log in to an existing Microsoft Account, or set up a new one.
If you have a Microsoft account, you can just sign in with your email address or phone number, and your password. If you don't have a Microsoft account, it's pretty simple: hit the "Get a New Email button" when prompted during your Xbox setup, or go to Microsoft's website to start account creation. You will just have to pick an email and a password, and set up an alternate method of contact for security. Once you've set up, you'll effectively have an Outlook account, an Xbox account, a Skype account and more, all under one simple single-sign in solution.
You'll also be able to use it to register with TrueAchievements to start tracking your Xbox game progression as well as engage with the community. It's one of many sign-in options on TrueAchievements, but it's one that we recommend.
Once your Microsoft Account is loaded onto the Xbox, the next job is to add your Gamertag, if there isn't one associated with that Account already.