Open iTunes. If you do not already have the program, iTunes is available as a free download from the Apple website (see Resources).
Click on the "iTunes Store" link on the left-hand side of the window. Sign in if necessary.
Click on the "Redeem" link on the right-hand side of the iTunes store page.
Type in the 16-digit code located on the back of your iTunes gift card. Click the "Redeem" button. The denomination will be credited to your account. If you do not already have an iTunes Store account, you will be taken to a page allowing you to create one.
Purchase music, TV show and movie downloads with your card simply by pressing the "Buy" button next to your download of choice. The cost will be automatically deducted from your balance.