First find a riding instructor. Then find a horse that suits you (is safe for you, not too big, well trained, preferably a gelding). Work only with this horse consistently so that he earns your trust and you earn his. Do everything....ride, groom, bathe, pick hooves. Spend lots of time with him. Learn how to read his body language; it speaks volumes. You will know when he is scared, relaxed, happy, ill, tired, or cranky. The time you spend on the ground with a horse will probably teach you more than the time you spend in the saddle.
I was just like you at 12. My mom couldn't afford a horse so I rode at a riding school for 4 years. Rode a lot of different horses, learned how to handle bucking, rearing....got kicked or stepped on a couple of times, etc. Sure it was scary, but I learned how to stay on, how not to get kicked, and how to keep my feet out of the way.
Until I had my own horse at age 37, I had no idea how special the bond of friendship can be with just one (when you're not riding a different horse every week). You'll gain confidence experimenting with horses other than your own. Then when you are ready to get one for yourself, you'll be amazed at how great it is to trust a horse completely.
But DON'T give up! I'm 41 now and horses have been the one constant interest I've had since age 3. Nothing has been more rewarding or inspiring.