I'm in high school. If I wanted to write a book someday, where would I start now?
1. Write something creative every day. This can be a journal, a blog, letters, stories, poems--anything, as long as you do it regularly and use your imagination.
2. Take any creative writing classes that your school or community offers. You may or may not learn how to write well there, but you will be "forced" to write creatively on a schedule. Also pay attention in your grammar and spelling lessons.
3. Read LOTS. While you read, pay attention to what works for you and what doesn't work. Does the dialog sound corny? Why? How would you write it differently? Does a character really intrigue you? What about the way the character was described really captivated you? Take notes. Then practice those techniques.
4. Write, write, write--wherever you can. Join the newspaper staff or set small writing goals for yourself. At Writers Digest, they have a daily writing prompt. Do those. Practice writing in different styles and genres.
5. Build a support group. Don't let anyone talk you out of writing. If your friends and family aren't supportive, then stop sharing your writing with them. Find a network of other writers who will support you and cheer you on. I like latterday authors and I think there are a couple of teenagers who post on that.
6. At some point, look at getting published. Local papers will sometimes publish columns from a teen perspective. Submit to magazines for teens. Read blogs and forums and books that talk about how to publish.
Most importantly, if you love to write, WRITE! Don't give up on your dream. And good luck.