Q: How do I use my iTunes card for in app purchases

Go onto iTunes/App Store. Scroll down to the bottom and there should be a button that says 'Redeem'. On your iTunes card, you should scratch off/peel the sticker in front of the code. Enter the code on your device and click Redeem in the top right hand corner. Then, go out of iTunes/App Store and into your app, and you can purchasee your item.

If you need more info:

iTunes Store: How to redeem a code