Dear Young Writer,
How awesome are you that you write? Let me tell you: more awesome than me. When I graduated from high school, I left behind my youthful writing - poems, illustrated serials featuring my friends, and psychedelic first-person stream-of-consciousness stories - for the "serious" work of becoming an engineer. I only allowed myself to rediscover my love of writing much later in life, after having my kids. Children are like your heart walking around on the earth: raw and exposed, new and vulnerable, and in wonder of the world around them. They tend to help you see the world anew.
Keep the child in you alive by keeping your writing close to your heart.
Write what you want. Write without fear. Don't let anyone tell you your writing is childish or unimportant - if it comes from your heart, it doesn't matter if it's silly or horrifying. It's legit, even if it's fanfiction. It's real writing because it's yours. You are unique in this world. Your stories have worth. Not just because writing exercises your imagination, something in serious short supply in the world. Not simply because clear writing requires clear thinking, another thing the world needs desperately. But because storytelling is as old as humanity and essential to being human. This is your original contribution - value it.
Keep your stories hidden in a journal. Or share them on Wattpad. Or self-publish your novel and give it to your grandma to keep on her shelf and brag on you to all her bridge buddies. However near or far you send your words into the world, don't be in too much of a hurry to become a professional writer. As I explained to my son, the twelve-year-old novelist, there's a difference between being an amateur writer and a professional one - and it's not that "amateur" writers are any less awesome. The Olympics are filled with nothing but amateur athletes, and they're the best in the world at what they do. But "going pro" means deciding to make a business out of your work... which, when you're still in high school (or younger) may not be the best place to take your writing. It can inhibit this wild and wonderful time in which you're learning to create. Protect that; cherish it. In the new world of self-publishing, it can seem like you must publish as soon as you've written something. You don't. Give your writing the gift of patience.
I've never been a patient person, so I know how hard this can be.
Write. Write a lot. Write passionately. Let yourself strive as an amateur. Don't worry about "going pro" with your writing just yet. That option will always be there. And the amazing thing about your stories is they will always be there, too. (Unless your friend's mother throws out three years of illustrated serials in an attempt to clean her daughter's room. Let this be an object lesson in backups.)
Keep being awesome,