How to redeem an iTunes Gift Card code?
Redeem an iTunes Gift Card on your iPhone, iPad, or iPod touch:

On your device, open the iTunes Store app.
Scroll to the bottom of the "Featured" section, then tap "Redeem".
Sign in with your Apple ID.
Tap "You can also enter your code manually".
Enter the 16-digit code starting with X, which can be found on the "My Game Card Page".
Tap "Redeem". After successful redemption, your iTunes account balance will be updated automatically.
Redeem an iTunes Gift Card on a Mac:

Launch iTunes.
Click "Sign In". Then enter your Apple ID and password.
Click your name. Select "Redeem" from the menu.
Enter your password again.
Enter the 16-digit code starting with X, which can be found on the "My Game Card Page".
Click "Redeem". After successful redemption, your iTunes account balance will be updated automatically.
iTunes Gift Card codes for different countries
iTunes Gift Cards are country/region restricted. If you try to redeem a gift card purchased in another country, make sure to redeem it in the iTunes Store of the same country. Gift cards cannot be redeemed outside the country of purchase. For example, a gift card purchased in the US cannot be redeemed in the Turkish iTunes Store.