I love that you love to read so much, and that you want to tell your own stories, too. Storytelling is an ancient pastime, a magical process, and a breathless escape from reality.
Start writing now. Don't think you have to wait until school's out, or you have your own computer, or you're an adult. You have stories worth sharing now, even as young as you are. It takes dedication and lots of hard work to write out an entire story, from start to finish. Many start; few finish. Practice being a finisher, right now.
Let other people read your stories, if you want. Share them with people that will love them. Ask each of those people for one idea of what you could do better. Get used to listening to other people's ideas, and deciding for yourself if you want to use them next time.
Keep reading books. Nothing helps you understand how to write better than absorbing the writings of those who have mastered the nitty-gritty of how sentences work. Pay attention as you read. At the end of a book, determine why you liked it so much (or didn't like it!). Those are valuable lessons you can apply to your own writing.
And finally, learn to shut out those voices (both yours and other people's) that tell you that you can't write a book. Surround yourself with people who will encourage and help you. Because you can write a book. Somewhere, floating in that head of yours, is a book only you can write. I hope you will.