The big idea
In my class, a "short story" is anything that is prose (that is to say, not poetry) and tells a made-up, or fictional, story. "Short" can mean anywhere from about a page and a half for fifth graders all the way up to ten or twelve pages, or even more, for eighth graders. When students ask me how long a story needs to be, I always say, "As long as it takes to do a good job."
Before you begin
To do a good job, a story must do these things:
have an interesting main character that the reader cares about;
have a clearly-described setting;
show the main character solving--or trying to solve--a problem.
How to do it
THE CHARACTERS
Stories need characters. Interesting characters are those that do and say interesting things. They have fears, friends, enemies, hopes for the future and favorite things they like to eat for breakfast, just as you have. In fact, I have a form that I often have students fill out before they start writing, on which they list these character details before they start writing. That way, the details are at their fingertips and they can slip them into the story at strategic points in order to make the writing more realistic. (NOTE: you need to USE these details where they would naturally show up in the story. You DON'T just list them at the beginning.) We also use those same details to generate the main problem of the story, as you will see below. Even if you don't use the form, however, you should spend some time thinking of these character details before you get going.
THE SETTING
The setting is the where and when of the story. You don't have to go overboard in describing the setting. If everyone in the story is riding horses and hanging out in a saloon, I pretty much get the idea that the setting is somewhere in the west in the 1800's. If the story starts with the hero riding the turbolift to the spaceport to catch the next rocket to Mars, I understand that I'm reading a science fiction story set in the future. Don't get things mixed up, though: When you have a medieval monk take a break from his work copying books in the scriptorium to go play Nintendo 64, I'm going to get confused.
A few well-placed details of setting can really set the mood. If the first scene opens in the dead of night with rain pounding against the windows and the wind howling outside, I'm ready to settle in for a really spooky story.
Don't ignore setting. It can help your story.
THE MAIN PROBLEMIf you don't have a main problem, you don't have a story. All stories have problems that must be solved: the Good Terminator has to protect Sarah Connor from the Evil Terminator; Jim Hawkins has to elude the pirates and bring home the gold from Treasure Island; Dr. Grant has to save the kids from the rampaging dinosaurs of Jurassic Park. Let me make this clear: STORIES ARE ABOUT CHARACTERS SOLVING PROBLEMS.
In class, we try to have big problems grow out of the details we have developed for our main characters. If our character wants to play professional football, what would be a good main problem for a story? Any number of things:
His girlfriend--the love of his life--will leave him if he continues to play football
He gets in an accident and has to fight his way back to playing condition
He gets in an accident and must learn to adjust to life without football
Congress passes a law prohibiting football
In order to liven up the game, the NFL replaces the ball with a bomb set to explode at some randomly-determined time.
If the main character you've developed is deathly afraid of spiders, what could your story be about?
Her house is invaded by millions of spiders
The only job she can find is as the spider keeper at the zoo
She finds herself slowly transforming into a spider
She crash-lands in the jungle and must live among the !Lwana tribesmen of the Amazon basin, for whom enormous spiders are a staple food
You get the idea. Big problems for stories are easy to come by. A good story is about an interesting character facing--and trying to solve--a big problem.
OTHER TIPS FOR STORY-WRITING SUCCESS
Unless you're writing a fairy tale, do not begin a story with the words, "Once upon a time" or its evil twins "One day," "One dark night," or anything similar.
You should also avoid, "Hi, my name is..."
Do use a lot of dialogue. People talk a lot, so characters in stories should, too. It brings stories to life. Just remember to make a new paragraph every time you switch speakers.
Get a good mixture of dialogue and narration, so the reader can visualize the setting, what the characters are doing, and so on.
Try beginning a story with either dialogue or action.
Don't be afraid to rewrite. If it's not great, fix it.