You need to follow, exactly, the instructions on this page when creating a new account : http://support.apple.com/kb/HT2534
If you don't use those instructions then credit card details will need to be entered before the account can be used to download any item from the store (you should be able to remove the card after entering it).
If you've already created an account and you are being prompted to review it then you could see if this post by mountaingoatgirl lets you do so without needing to enter credit card details : https://discussions.apple.com/message/24303054
Creating an account with 'none' via a computer's iTunes : https://discussions.apple.com/message/24321860