You just need to redeem your Gift card on your account, here's how:
1) Make sure you have the latest version of iTunes. You can download it free of charge:
2) Open iTunes and click iTunes Store on the left side of the window.
3) Under Quick Links on the right side of the window, click Redeem.
4) Enter the code in the Redeem Code ﬁeld and click Redeem.
If you haven't yet set up an iTunes account, you will now be given the option. If you do not wish to enter a credit card number, click the None button on the Payment screen.
Once your Gift Card is successfully redeemed, your store credit will appear next to your account name in the upper-right corner of the iTunes Store window. You will not need to redeem your Gift Card again when you want to make another purchase. Each time you make a purchase, funds will be deducted from the credit until the credit is used up. If you run over the credit amount, iTunes will ask you to enter a credit card number.