This is for Zak, and anyone else who dreams of being a published writer one day. Why? Because I was like you once - a kid who wrote and dreamed of being published. (There was no internet back then, in the cyber dark ages, and it was a great deal harder to find out anything about how best to do that. So you are much luckier now!)
Read what you love to read, as widely as possible. Never neglect your reading. As you grow older, you may think you don't have time to read -- but there always is time, even if it's just a few moments on a bus, or while waiting for a friend. Only by reading can you learn so much so easily and pleasantly about how to write.
For fun. Write the stories you'd like to read. No one gets to be good at something without practice. Think about people who play the piano well - they practice all the time. It doesn't matter if no one but you reads what you write - you are learning something every time you type a paragraph.
3. Things like spelling and good grammar are important.
Take the time to spell check and learn grammar rules. Only once a writer understands the rules, does s/he understand how rules can occasionally be broken.
4. Read books or articles about writing
You may find many interesting things on the web pages of some of your favourite authors.
5. As you grow up, seek the company of other writers.
We learn from one another. We support one another. We crit one another's writing. If you are a fantasy/science fiction writer, then find ways to join your local SF organization or go to local meetings, readings or conventions.
6. Don't be in too much of a hurry...
No one gets to play professional tennis and win trophies without years of practice. Writers usually don't get their first book professionally published until they are are more than thirty. Of course, there are exceptions, but there's no guarantees that you'll be one of them! Writing short stories can be a way to get your name out there and even to earn pocket money.
7. ... but don't lose sight of your goal.
Teenage years can be a tough time to write. You'll have lots of things to write about, but little time to do it. Keep writing nonetheless, even if it's only occasionally. And don't despair if your writing output plummets. You are learning valuable things about people. You are observing life at its most wonderful, and its most stressful and sometimes, sometimes even -- unfortunately -- at its most tragic. These are the memories that will make you a fine writer one day. Remember how life feels to you. Live. Think. Remember. Write. Consider keeping a private diary.
8. Buy books when you can.
If you don't have the money, tell your friends and/or family that you want books for presents. Why? Apart from the joy of owning books, you will be ensuring that the book industry remains a healthy one. After all, they need to have money to buy your manuscript and publish your book one day! It is odd how many would-be authors don't support book publishers in this way.
9. Looking ahead: Don't plan on studying creative writing at college/university.
You'll learn more about the things you need to know studying almost anything else - veterinary science, literature, geology, history, sociology, anything at all... You need to experience life. You need to have a job that earns you a living so you can write and hone your skills in your spare time.
10. Hold onto the dream.
Anything worth doing is worth doing well, and it takes time. Don't lose heart.