Go to http://www.apple.com/itunes/, and download iTunes onto your computer if you do not already have it.
Open iTunes, and click on "iTunes Store" on the left side of the screen.
Go to the "Quick Links" window on the right side of the screen and click "Redeem."
Enter the code from your prepaid iTunes card into the space provided and click "Redeem." This will credit your iTunes account with the amount of money on the card.
Purchase items from the iTunes store against the balance of your card.