Hello, Mel0524.
Thank you for visiting Apple Support Communities.
Here are the steps on how to redeem an iTunes Gift Card in unfamiliar with the process.
Redeeming iTunes Gift Cards and content codes
http://support.apple.com/kb/ht1574
If you are having issues redeeming an iTunes Gift Card, here is an article that you may find helpful. See the section labeled Contact iTunes Store Support.
If you can't redeem your iTunes Gift Card or code
http://support.apple.com/kb/HT6021
Also, here is some helpful information regarding how iTunes purchases are billed.
iTunes Store: How iTunes Store purchases are billed
http://support.apple.com/kb/HT5582
Cheers,
Jason H.