ITunes gift cards apply credit to your Apple ID account, so you don't need to wait until you're ready to shop to enter the code. Just log in to your account, redeem the card and you'll find the store credit ready for use whenever you next buy music, movies, books or apps. As long as you have enough credit in your account to pay for a purchase, you won't need to enter other payment information. Enter your gift card either on your iOS device or in iTunes. No matter which you use, your credit works in both locations.
Redeem on an iPhone, iPad or iPod Touch
Open either the App Store app or the iTunes Store app in iOS. Gift credit works across both storefronts, so it doesn't matter which you use to redeem the card. Scroll to the bottom of the main tab (Featured in the App Store or Music in the iTunes Store) and tap Redeem. Enter your Apple ID password to verify your account.
Tap Use Camera and aim the camera at the code on the back of your gift card, located beneath the scratch-off label. If the camera can't pick up the code or you get an error message, tap You can also enter your code manually, type the number by hand and tap Redeem.
Tap Done to finish, or tap Redeem Another Code to add a second gift card. Any time you want to use your gift credit, just tap an item's price in either storefront and tap Buy. ITunes automatically uses your gift credit rather than charging your credit card, even if you have one on file.
Redeem in iTunes
Open the Account menu in iTunes 12 and click Redeem.
Enter the 16-character code found under the scratch-off panel on the back of the gift card and click Redeem.
Click Done to finish redeeming cards or click Redeem Another Code. Head to the iTunes Store whenever you're ready to shop, click Buy on an item and the funds automatically come out of your gift credit.