In order to use the iTunes store, you need to register with Apple and create an "Apple-ID". The registration process will at some point want to confirm your email address and thus send you an email with a link.
When you click on the link, you get to a page that tells you to enter your "Apple-ID" and your password to confirm the confirmation.
Nowhere you have been told up to that point what that "Apple-ID" is supposed to be, nor will Googling help you (or at least me).
The "Apple-ID" is plainly your email address that you were asked to confirm in the first place.
Tomáš Pospíšek, 2013-02-24