If you're 15 or younger, you can enter the Children's Fiction Writing Awards - and get published!
Winning stories will be published in an anthology. Your biography and photo will be in there too, so start practicing your Top Model smile. But hurry - the deadline is October 10th.
In the 12-15 year old category, stories need to be at least 3000 words (about 6 pages).
The stories must fall within one of the following genres: Science Fiction, Fantasy, or Mystery. So if your story has vampires, spaceships, or secrets, you're good.
Check out the Calgary Book Fair site for details.
Writer's block? Check out these collections to get some ideas:

Lay-Ups and Long Shots: an Anthology of Short Stories
Angry Management by Chris Crutcher
The Starry Rift: Tales of New Tomorrows (includes stories by Scott Westerfeld and Neil Gaiman)
Firebirds Rising (includes stories from Tamora Pierce and Diana Wyne Jones)
Prom Nights from Hell (includes stories by Stephenie Meyer and Meg Cabot)