First, log into the iTunes Store and look at your purchase history. If you do not see the purchase listed there, it's quite possible that the charge was a fraudulent submission directly to your bank and did not go through the iTunes Store at all. This has been often the case with such fraudulent charges. You will need to work with your bank if this is indeed the case (and you should report the charges to your bank as unauthorized in any case).
If you see the purchase listed in your Purchase History, remove your bank card information and change your iTunes Store password, if you haven't done so already. Then go here:
to report the issue to the iTunes Store.
I would advise never using a debit card for any site/company where they keep your account information for future purchases. You are much more risk than with a standard credit card, since a debit card immediately draws out your money and hence is much more painful and difficult to handle should fraud or error occur.