Where can I get itunes gift card code? 'Cause I can't sign in to iTunes.

When I signed in to my app store and itunes in my new iphone, something appeared and noticed me that my Apple ID was not used in itunes store and tap "Review" to sign in. So i tapped "Review" and made my way in an application form (billing info, I guess). I filled it up except the one that needs a code of itunes gift card. But I dont have any gift card code, what should I do and where can I get it because I cant get apps from my app store?! Anyone please reply.