Read A Lot: There's no easier way of absorbing writing skills than by reading. Don't limit your reading to novels. I used to read plays all the time in junior high and high school, and learned a great deal, without realizing it, about dialogue and structure (I learned a lot also from watching old movies). Poetry teaches you style and vision. Biographies (which I read a lot of as well) teach you about how people, and not just famous ones, behave and grow. Newspapers, magazines, and the internet, show what people are interested in.
Write A Lot: Not just the stuff you have to write for school. Write for your own pleasure. Writing is the same as skateboarding or playing the piano. You get better through practice. William Shakespeare knocked out a lot of plays before he could write King Lear.
Learn Grammar And Spelling: They're boring, but essential. And don't count on the computer to know everything for you. A sentence like: There, they're their own worst enemy, can give even the smartest computer a migraine. For that matter, so could: It's its own worst enemy. So don't be your own worst enemy, by trying to get away with mistake ridden manuscripts.
Pay Attention To People: Watch your family, your friends, your teachers, anybody you have contact with. Ask yourself why they behave the way they do. The better you understand people, the easier it is to create characters. And even if you decide your life work is to write textbooks about rocks, people will be your target audience (so few rocks read these days). The best non-fiction writers know how to make their subject interesting to people, just the same as fiction writers.
Listen To Praise And To Criticism: You can learn from both. Don't assume all praise is accurate or all criticism is. But don't reject praise or criticism automatically. I've learned from people who like my writing, and I've learned from people who don't.
Try To Get Published: Getting published doesn't necessarily mean getting your novel published by a big time publishing house. That's hard under any circumstances, and big time publishing houses these days are struggling, the same as many other industries. But if your school has a paper or a magazine, or your local newspaper has a teen section, or you know of a magazine (print or online) that is looking for submissions by younger writers, go for it. There's something very encouraging about seeing your name in print.
Write About What You Like Best: If you love to surf, then write about surfing or surfers or ocean waves. If you love fashion, then write about fashion or people who love fashion or people who don't love fashion. If you're like me, and what you love best is figuring out how families would behave during hard times, then write about families in hard times.
Write About What You Know Best: Maybe what you know best is surfing. Maybe what you know best is fashion. But maybe what you know best is how sisters or friends or parents fight. Maybe what you know best is how you feel when you fight with your sister or your friend or your parent. You can know feelings just as well as you can know facts, and they're both great starting off points for writing.
Be True To Feelings, Not Facts: When I was a kid, I was scared of going to the dentist. When I was a grownup, I wrote a book called What Do You Do When Your Mouth Won't Open, about a kid who was scared of speaking in public. I've never been scared of speaking in public, but I understood irrational fear. So I took feelings I'd had and gave them to my heroine. The great thing about fiction is you can put your characters wherever you want them, but wherever you do, you must be sure their feelings will seem real to your readers.
Have Fun With Your Writing: I always tell people, I'm my own biggest fan, because I write the stories I would most enjoy reading. There's not much point being a writer if you don't have fun with it. But if you do have fun, then it's a wonderful job, or hobby, or stepping stone to whatever the next step in your life will be.