how to buy from itune store

Go into the Itunes store and select an item to purchase, if you have a Credit Card associated with your account, or you have redeemed an iTunes gift card and you have enough balance to cover the cost of the App, you can then purchase it and it will begin download.

iTunes Store: How iTunes Store purchases are billed

http://support.apple.com/kb/HT5582

It works essentially the same for the App Store.