In the Uk, for £39.99 you get a year's subscription, smaller versions (not demos) of MotoGP Online and Whacked! and the XBox voice communicator. Unfortuanatley, Microsoft are accepting only credit/debit card. I don't know what a debit card is, I'm afraid
I don't think you actually need to be 18 to get a credit card anyway. Why not pay a visit to your bank and ask?
Remember, you will have to pay for the monthly fees that come with broadband. If you don't already have it, make sure you get as good a deal as possible and it is recommended to have a 500kb/sec tansfer rate or higher (256kb/sec being the minimum for XBox Live). Microsoft are doing deals to bring down the prices of broadband.
[EDIT] Oh yeah, and you'll probbaly need your credit card for when the year's subscription is up and you pay for the next year. And some games (only some, not all) provide downloadable content in exchange for moolah. The credit card would probably be used for this. Most games like Mech Assault and Splinter Cell shouldn't cost anything for downloadable content though.[/EDIT]