This could actually happen for a number of reasons:
Have a look at the article below in Greek or English,I am hoping it will help you :-)
Also kindly note that:
1.The FULL credit card information must be correct (like the bank has it on their system)
2.The CVC (last 3 digits) of the card needs to be correct
3.Sometimes trying another card from a different Bank may help
4.Trying another device like a Windows computer/iPad/iPod etc may help
5.The card must have at least 1.50 in your local currency ( this will be refunded to you in a few days)
6.If you don't want to buy something you should have the option of selecting "none" as a payment option.
Hope this helps you.
Don't forget AppleCare is at your disposal