A short story is a type of prose fiction.
Prose simply means it is written in sentences and paragraphs, and is not a poem or play-script.
Fiction means the story is made up, though of course you can base your story on something that has really happened.
A short story is not a cut-down novel. In a novel the author has time to develop characters and show us many events in their lives. A short story takes characters at an important point in their lives and gives us a snapshot of a significant moment.
Before writing a short story, consider the narrative perspective you are going to take. This is usually done in the first or third person.
If you want to tell the story from the point of view of one of the characters, you can write in the first person, using 'I'. This puts the reader inside the mind of that character, seeing things as that character sees them.
The character you choose to tell the first person narrative is important. A story about bullying, for example, will be different depending on whether you tell it from the point of view of the bully or the victim.
This is where you tell the story from the point of view of someone not in the story, using 'he' and 'she' throughout. This means the story can be told from a more objective point of view.
You need to decide which point of view to take, depending on which is best for your particular story. Sometimes if a story is not working it can help to change it from third to first person narrative or vice versa.