Sorry to tell you, but the short answer is: you don't. Apple requires you to have some form of payment on hand when creating an iTunes account.
Apple needs to have a way to bill you for any purchases you make at the iTunes Store. Even though you can download lots of free content at the iTunes Store, there’s nothing to prevent you from buying content, too, so Apple needs a way to bill you for any commercial content you get.
There's on way around this, though: If you don’t have a credit card, but do have a PayPal account, you can choose to have your purchases billed to that account instead. And no, just having a gift card won't do it, either.

