You'll get the box with a registration code on it. Go to the website that it tells you, enter the code and your serial number and boom you're all set. Occasionally your purchase date might not be confirmed, and they'll email asking for a copy of the receipt.
Suggestion. Try looking at BHPhotoVideo. They sell AppleCare at a really good discount and is tax free for most people. 🙂. Registration works the same way.