You can use your iTunes Gift Card in the iTunes Store, Mac App Store, and iBooks Store. Just sign in to a store with your Apple ID. Click Redeem in the Quick Links section of the store, then enter your card number.
If you received a gift card through email, just click Redeem Now in the email. If you’re not already signed in to iTunes, you need to sign it.
When you redeem a gift card, the amount is credited to your Apple ID account. Each time you purchase content, funds are deducted from the credit until it's gone.