Oh honey, I understand your dilemma.
As an old horsewoman here's what I recommend.
1. Make the very best grades in school that you can.
2. If you need help with grades, speak up about getting a tutor so you can make good grades.
3. Do your home chores without being asked.
4. Be responsible about who your friends are - make sure they're nice, polite, responsible kids just like you are.
5. This summer, try to volunteer at a local stable (if you're old enough and there is one near you) in exchange for lessons and riding.
6. Learn as much as you can from other horse owners, internet, magazines etc.
TAKE LESSONS! there is nothing better than lessons for a new rider - you learn how to ride properly. Also, after a few weeks of lessons, you will know if riding is the sport for you.
Now then. Think of this. Buying the horse is just the beginning. There's the upkeep - Feed, boarding, vet bills, worming, hoof care, equipment, hauling, show fees, clothing, boots, helmet etc. Depending on your area that can run into thousands of dollars per year. So buying a horse is no small consideration.
For Mom & Dad - consider this. I once had a parent come to me and insist that I do everythin I could to keep his daughter happy with her horse. That way he knew that his daughter was safe and sound, getting good and dirty - and having a ball with her horse. AND she was NOT at the mall with a bunch of hooligans or out with some punk doing drugs or getting pregnant.