buy grift card

This article will give you step-by-step instructions on how to redeem an iTunes gift card.
Steps
1-Make sure you have downloaded iTunes. You can do this for free from the Apple website.
2-Create an iTunes account. This will ask you for some personal information. It is important that you enter your real information. You already have an iTunes account if you have a .Mac membership.
3-Open iTunes Then, click on iTunes Store in the pane at the left side of the screen.
4-Confirm that you are signed in as yourself. In the upper-left corner of the iTunes Shop window, verify that the button says your e-mail address.
5-In the Quick Links window in the right-hand column, click Redeem.
6-Scratch off the latex covering on the back side of the gift card.
7-In the resulting screen, enter the 16-digit code and then click Redeem.
8-Once you've successfully redeemed your gift card, the credit amount will appear next to your account name in the bottom-left corner. Each time you purchase items, iTunes will deduct funds from the credit until you run out.

Tips
*iTunes gift cards are good to use if you do not have a credit card, or if you don't want to give your credit card information to Apple. Simply purchase a gift card at a store for yourself and redeem it using the above steps.

Warnings
*Scratch off the code gently! The numbers will not be visible otherwise.
*Make sure it is your account you are crediting, not someone else's.

source:
http://www.wikihow.com/Use-an-iTunes-Gift-Card