It is a lot easier to redeem your iTunes Gift Card from your iPhone and iPad than on the computer. It allows you the extra useful option of snapping the code on your gift card, so you wouldn't need to type out the long code.

To redeem your iTunes gift card on your iPhone or iPad, launch the iTunes app. Scroll down on the first page that comes up and you will see Redeem.

Tap on it and you will be asked to enter your login details. Finally, you will be presented with the option to type in the code or snap the iTunes card with your camera.