Q: how do i buy a song using an itunes giftcard

Once you redeem your iTunes Gift Card, the full amount appears next to your account ID name in the upper-right part of the iTunes Store main window as a credit. Any time you buy an item, the purchase price (+ tax) will be deducted from the balance.  The store will charhge the gift card balance until it's depleted, then charge your credit card.  Read more at http://support.apple.com/kb/HT2736#2