You don't have to have a credit card to use the iTunes Store for free items but it is normally used at some point to validate your address so that, for example, you don't try to access the store of a different country.
There are some hoops that you can jump through as you create your iTunes account (Create an iTunes Store, App Store, or iBooks Store account without a credit card or other payment method) that workaround the need for a card, otherwise once the account is set up iTunes may still expect you to provide one, which you can subsequently remove. If you don't have or want to use a credit card at this time you can try this method...
- Enable the menu bar if needed with Ctrl+B (Applies to Windows only)
- Sign out of the iTunes Store from the Store item on the menu bar
- Go to iTunes Store home page
- Go to Quick Links > Redeem
- Sign in with your Apple ID
- Go through the screens, accepting the EULA, etc. You should arrive at a Review Payment screen where one of the options is None - select this and continue... you shouldn't actually need to redeem a new voucher/card
I've adapted the steps slightly from this post. I haven't tested the exact scenario since my account is already set up so I get straight through to the redeem code screen but you should be able to choose and confirm None as the payment method.
tt2