In school I wrote reports and essays, that kind of thing, but I didn't actually start writing stories and poetry until I was in college. Back when I was a kid, we weren't being encouraged in school to try writing our own stories like kids are today. I think it's terrific how many kids are seeing themselves as writiers, and are creating their own stories and poems. I really do. You learn by doing it, just like you'd learn to play a musical instrument or learn to play basketball.
Writing begins with reading.
I think of reading as the rain and snow that filters into the underground water table of your imagination. Writing is like sinking a well. If you're a reader you'll have a reservoir of good ideas and a feel for language that will prove invaluable as you start to write. You get better by doing it. Get a rough draft written that you can work with, and revise it later.