I need help with my MasterCard it won't work

If it's a debit card then I don't think that they are still accepted as a valid payment method - they are not listed on this page and there have been a number of posts recently about them being declined

If it is a credit card then is it registered to exactly the same name and address (including format and spacing etc) that you have on your iTunes account, it was issued by a bank in your country and you are currently in that country ? 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 and see if they know why it's being declined :http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page