Use an App Store & iTunes Gift Card on PC

If you received an iTunes Store Gift Card or promotional code, you can use it to buy songs, videos, or audiobooks from the iTunes Store.

Set up an Apple ID (required before you can redeem anything in the iTunes Store).
In the iTunes app on your PC, choose Account > Redeem, then follow the onscreen instructions.
You don’t need to enter credit card information to redeem only the amount of a gift card.

Your balance appears in the top-right corner of the Store window.

You can use a gift card only in the iTunes Store for the same country or region in which it was purchased.

Helpful?

iTunes User Guide