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. Find a partner who also likes to write and read each other's stuff, provide encouragement, make suggestions. Remember that both readers and writers are imagining what it's like to be someone else, so write with the five senses.