Make sure that you're using a true credit card, not a debit card, and that the card was issued in your country and has a billing address in your country, and that you're using the iTunes Store of your own country. You cannot use a credit card from another country, a debit card, nor can you use the iTunes Store of another country.
If all that is correct and you're still having problems, go here:
http://www.apple.com/support/itunes/contact/
and follow the instructions to report the issue to the iTunes Store.
Regards.