What type of card are you trying to use as your payment method, credit card or debit card ? Debit cards don't appear to still be accepted in all countries, and even where they are it seems (from posts on here) that not all work - this page includes :
You may be able to use other payment types in your country, like debit and Maestro cards.
which implies that they are not accepted in all countries, and there have been a number of posts about them being declined.
For a card to have a chance of being accepted it needs to be registered to exactly the same name and address (including format and spacing etc) that you have on your iTunes account, and have been issued by a bank in the country where you and your iTunes account are. If it is then you could check with the card issuer to see if it's them that are declining it, and if not then try contacting iTunes Support (these are user-to-user forums) and see if they know why it's being declined (but you might not be able to use a debit card) : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then Account Management