Writing is not something you can never do for anyone but yourself. If you do, you're going to get dejected by criticism, discouraged when your readers just don't get what you're saying, and heartbroken when the rejection slips start coming. And they will. No matter how good you are.
Writing is a cold business. Editors don't care how many years you spent writing a novel. They only care if it fits in with their current market projections.
Writing is also a gift. Think of how many people in the world didn't receive enough of an education to write. To some, it would be a blessing beyond words to sit and write a half page letter to a loved one because they simply can't. Some are imprisoned by their own bodies and prevented from writing due to injury or ailment.
I've heard all my life that if you can possibly see yourself doing anything else with your life, then don't write. I can understand why. But I also understand the passion to write, the desire to captivate readers and transform their thoughts and emotions with nothing more than some ink and paper and a few hours of their time. Heck, God spoke to us through his Word, written in the Bible. That alone should be enough to convince us of the awesome and inspiring power of the written word.
At fifteen, you've got a lot of years ahead of you. You've got a few behind you, too, to draw from. Go out and live life. When I was sixteen, a teacher asked me what I wanted to do and I said right off that I wanted to write. He said good, I might be good at it someday, but only if I lived long enough to have something to write about. Living, I've learned, has nothing to do with age and everything to do with the quality of life and the accumulation of experiences.
All you have to do is write what's on your heart. What's in your heart. Pure and unadulterated. Don't do it for your mom, don't do it for your boyfriend or a potential vast audience of adoring readers, do it for yourself. Aside from that, read as much as you can, particularly about the craft of writing. Stephen King has a good book on writing (at least the back half is about writing, I never bothered to read more than 40 pages of the first half). Study the elements of style and understand how to use the language. Pay attention in school and take advantage of your classes so you'll have knowledge to pepper your writing with. Hook up with a community of writers, either online or in your school or community. Read some good books and get inspired.
Writing is like a lot of things in life. Either come into and give it everything you've got or pack your bags and go home. Ultimately, only you can decide what is the right path for you.
P.S. If you ever get the urge to go into engineering, it's a good sign that you're meant to be a writer and living in denial. Take it from someone with experience.