You need to do a couple of things. First, move your games, saves, accounts and whatever you need to your new xbox. You can use a transfer cable, which you can buy now, or just use a usb drive. If your firmware on your old box is up to date, you can use up to a 16gb usb drive (flash, or a spinning external hd,) just make sure nothing you need is on the drives, as your xbox will wipe and reformat it once you begin the "configure new device" process. Then from the settings>memory menu, copy to the external drive then copy to the new hard drive. If you don't have too much DLC or Arcade games, this should be quick. If you have a lot of stuff, consider that it might be quicker just to re-download on your new xbox sometimes. That's up to you.
The second step is to tranfer your licences to your new xbox, using this web site:
http://www.xbox.com/en-us/support/sy...ensemigration/
that way anything you've purchased can be used by any gamer on your new xbox, and not just when the account that purchased them is logged in. All you need is your new xbox's Serial number. For some reason, you can only do this once per year though.