Do you have an iTunes Card that you want to redeem in to your iTunes Store (or App Store) using your iPhone.
Here is how you do that:
In This tutorial we are assuming that you are using your iPhone and are logged in with your Apple ID.
If it´s not your iPhone or Apple ID you should not use this method.
Open the App Store on your device. In App Store, tap Featured from the bottom menu bar and scroll down the bottom of the page. There you must tap Redeem button.
In the text box, enter your iTunes Gift Card code, and tap Redeem.
(The card code is printed on the back of the card).