Purchase the iTunes card at a reputable retail store. At purchase time the cashier will swipe the card to activate it in addition to ringing it up.
Open iTunes and go to the iTunes store, found in the left menu area above playlists.
Click on "redeem" in the right menu area of the iTunes store under "Quick Links."
Scratch off the back of the card to reveal the code. Enter this code in the box under the words "Redeem Code" exactly as it appears. Then click on the "Redeem" button. If it tells you the card has not been activated then you must return to the store where it was purchased. This happens if the cashier does not properly activate the card by swiping it. Bring your receipt and ask for a manager; he will be able to properly activate the card for you. Once it is properly activated, go back to step 2.