Zero's strategy is your best bet as long as you only want to use it on iTunes.
Most online retailers require verification on a card in order to put a charge on it. You have to call the company that issued the card (or visit their website) and enter your name and mailing address.Then you enter that information as the billing address when you make a purchase.
If it's a Vanilla Visa card, I'm afraid you're probably SOL. I bought one of those a couple years ago, and for some reason they stopped registering cards. Calling didn't work, and they took the registration site down. It never worked on iTunes, Steam, or NewEgg.
If you want to buy things regularly on the Internet without the hanging debt of a credit card, start an additional free checking account with the bank, get a debit card for it, and keep a low balance. That way you can only spend as much as you put on it, with no worrying about monthly bills and high interest rates.