There an article relating to this writtin by the credit watchdog in the UK here:

http://www.endorsedcards.com/blog/itunes-accounts-without-a-credit-card/

It suggests getting either a disposable prepaid card or a debit card.

In teh UK there are very few options, but in the US there are a lot of ways to get a prepaid card as long as your parents buy it for you and they have a SS number.

Hope that helped.