Writing a short story is easier when you remember that there are basically four parts to every story.
First you need to have characters (people or other creatures) that interrelate in the story. Every character is important to the story in some way. If you have an uncle Bob showing up in your story, there is a reason for having him there. In other words he contributes a necessary part to the story.
The story happens in a place. That could be a school, or a home, or a park or even a different planet. You could just outright describe the place but it is better to have the place unravel before the reader as the story goes on. Here is an example:
Sophie looked up through the frost coated branches and saw the tattered kite flapping in the cold breeze. 'I wonder how it got there so deep in this forest,' she thought out loud to no one in particular.
Stories have activities. These happenings move the story along and keep the reader interested. In a short story there is a main activity rather than many activities such as a long story might have. A main activity might be the breakup of friends, or a move to a new city, or the solving of a mystery.
The main problem in the story is called the conflict and is the thing that keeps the reader interested in reading the story. He wants to know how it all turns out in the end. This could be a quarrel or battle. It could be some inner struggle within the person himself. Or even barriers that have to be overcome in order to obtain some goal.
EXAMPLE You might write a story about two boys of the same age(characters) going on a camping trip (activity) to a nearby lake (place). While camping, one of the boys disappears after an argument with the camp counsellor (conflict) and his friend goes out to find him.
EXAMPLE Or a story about a girl who has become friends with a boy (characters) that other teens don't like and she has a struggle within herself (conflict) as to whether she should hang with him or not. The story takes place just before the highschool prom (activity) in a small town in Ohio (place).
So the next story you write (or even read) check for these four things: character, place, activity, and conflict.