Congrats on having the foresight to start managing your money well from the very start! I'd recommend you go get the book
All Your Worth from the library. This book gives a good philosophy for how to allocate your money. The principal idea is to spend no more than 50% on your needs, 30% on your wants, and at least 20% on savings. So for a 15 year old, I would assume you are still living at home and most of your needs are taken care of by your parents. Here's what I would suggest you do with each paycheck:
Set aside 50% toward buying a car.
Allow yourself to spend 30% on fun stuff.
Set aside 20% in a savings account. This account is going to be the cornerstone for your future wealth.
Pick a long-term goal (5 years or more) for your long-term savings. Perhaps you'd like to take a trip to Europe when you are 20, or put a downpayment on a house when you are 25. When your non-car savings reaches $250, buy your first mutual fund (the folks here can help you pick one out.) After that, put the 20% into the mutual fund instead of savings. I myself had a mutual fund when I was in high school that I contributed small amounts to regularly, and think that it did a lot toward putting me on the right path. See my post here:
x-post from Consider starting your teenager in a Mutual Fund: Zetta's Striving to Get Rich Slow