Actually there's (at least) something called Mastercard Young, where the parents can make cards for the kids (6-14 years!).
You are right, it's not a credit card, it's a debit card, but somehow I expect most kids don't really understand nor care about the difference.
However, OP, if you are 14 years old, best is to ask your parents nicely to buy for you. And that's especially because Bitcoin is, after all, a highly volatile asset and a risky investment, so you may have to rely on their judgement.
I believed that kind of cards has restriction since it's designed for kids. I doubt they can make transactions in terms of purchasing BTC or related services if ever.
The best thing for OP is to ask the parents instead. Don't make this secret after all. Surely, the money will come from his parents. Giving suggestions might lead the OP to not telling the truth. Or maybe if OP wants to surprise his parents in the future that he now lots of money, ask some trusted seniors instead. He will be safe after all since purchased BTC will be stored at his own wallet and he will accompany the account owner while buying BTC. I just don't know on what level of knowledge does OP has now in terms of buying and storing BTC.