In no particular order, here are some hints to help get you started:

• start your short story at an exciting part or near the end of the story
• keep your story under 1,500 words.  Under 1,000 is even better.
• limit your characters- have maybe less than 5 characters- more characters means more explaining what everyone is doing.
• always choose suspense and action over death and bloodshed
• write about characters you know, like kids your age- you know how kids think, avoid writing about adults
• make up your characters- don't use friends' names or we will think about those kids instead of your characters
• write about a place you know well and limit your story to one or two locations
• limit your story to one day, a few hours is even better- more than that and characters have to eat, go to the bathroom, sleep, tell parents where they are...
• create one big problem, have a solution that doesn't work and adds to the problem, then have a solution that works
• a clever story doesn't end with "to be continued," it implies that more and different problems are to come in the future