There are two major and easy ways:
Use your iTunes Store app on your Apple device:
1. Press the Music tab on the bottom menu
2. Scroll to the bottom until you see a Redeem icon
3. Press Redeem
4. Enter the iTunes gift card number in the given text field and click redeem(top right corner)
You can also just use the App Store app:
1. Open the App Store
2. Click on the Featured tab on the bottom menu
3. Scroll to the bottom until you see a Redeem icon.
4. Press Redeem
5. Type in the iTunes gift card number, click redeem and that is it
6. Now you can use your App Store balance for iTunes