If you have an outstanding balance for an iTunes Store order on your account, you can pay it by either adding a new credit card to your account or if iTunes Gift Cards are available in your country, by redeeming enough store credit to pay the balance in full.
To change your account or payment information, follow the instructions in this article:
iTunes Store: Changing Account Information
http://support.apple.com/kb/HT1918
This article explains how to purchase and redeem iTunes gifts:
iTunes Gift Options
http://support.apple.com/kb/HT2736