If you're 15, you should be able to go down to your bank and get a prepaid visa.
It's like a visa for under 18s, only you can't put money back on it once you've used it. There may be an extra fee for buying it. For instance, when I got a pre-paid visa to get blockland, I put £20 on it, and I had to pay £22. Just use the visa on the blockland website then. That's what I did.