Either you already signed up for this long ago, or someone else accidentally typed in your email address to use on accident.
If you have another email address, go ahead and sign using that one here:
... and if you don't have another email address, just go here and sign up for a new one:
Just click the "Create New Account" button.
Then go here:
...and sign up using your new email account.
And if you experience any other problems, there's a full list of help here:
Enjoy the gifts!