How did you create the account ? Unless the instructions on this page are followed when creating an account : http://support.apple.com/kb/HT2534
then credit card details will need to be entered before the account can be used to download any item from the store.
You could see if this post by mountaingoatgirl lets you review your account without needing to enter credit card details : https://discussions.apple.com/message/24303054
Creating a new account with 'none' :
computer's iTunes : https://discussions.apple.com/message/24321860
iOS 7 : https://discussions.apple.com/message/24700173