If you have a story to tell but aren't looking to write an entire book, a short story may be the best format to approach. Writing short stories can be a tough task since there aren't as many words available to develop characters and allow the story to mature, but good writers can make it happen.
Short Story Word Count
Though it's important to keep the ideal word count for short stories in mind - 1,500 to 30,000 words - it's also important to tell the story without feeling confined by the number of words you have to put into the story. If your story runs long, go back and edit out what's not vital to the story. If your word count is quite low, yet you truly feel your story is well told, you shouldn't add more words if they won't make the story any better.
To Chapter or Not to Chapter
Very short stories likely don't need chapters unless the narrative calls for them with the necessity for a huge change in the story. Used correctly as a literary device, thoughtful chaptering can help push the story along and help the reader navigate the story better.
If you decide not to use chapters, don't allow the story to become one big chunk of stream-of-consciousness text (unless, of course, it's what you're aiming for deliberately). Modern readers - especially those online - simply do not do well with large blocks of text and you may lose their attention as a result.
Short Story Outline
Though it isn't mandatory to craft an outline before writing a short story, it does help to know where your story starts and how it ends - and have an idea of how you'll get there. Don't panic if your story takes a turn you weren't expecting; don't fight it in an attempt to stick to the outline. That's the great thing about outlines: they can be revised if necessary.
Let the Characters Guide You
If you know your characters as well as you should, you will know if you're trying to force them into situations that are "wrong" for them. Pause occasionally to review the story so far and ensure your characters are behaving in a way that is consistent with how you've presented them.
If you feel compelled to make a character do something that doesn't make sense to the story, find the motivation behind the incongruousness; it may prove to be an epiphany for you as a writer that the story needs to go in a different direction and perhaps the characters aren't who you think they are.
Show, Don't Tell
Your story should reveal itself to your readers without you (as the writer) or the character (as the narrator) spelling it all out for them. Your short story should not be an explanation of something that happened, but rather an event that's observed by the readers, allowing them to come up with their own conclusions about the story.
As a storyteller, you may want to pad up your short story with a parade of colorful characters, but the truth about short stories is that you simply don't have the time to develop all these characters sufficiently before the story comes to an end. Trying to stuff too much into the story will make it a difficult read.
Know Your Reader
If your story is for submission to a publication or a contest, these factors will dictate what type of story it will be. Pay close attention to the rules set forth by the accepting entity. Even if you're convinced the story is pure gold, yet it exceeds the word count designated beforehand, there's a good chance your story will be discarded.
Stories written for fun should be written with the reader in mind as well. Will your reader find the story engaging? What emotion do you want to elicit? What point do you want to get across? Keep this in mind throughout the writing process and steer the story toward this goal consistently.
Before It's Ready
While it's true that the point of your first draft should largely be to get the story written and therefore you should not worry as much about grammar and spelling, your second draft and any subsequent polishing of the story should include such fixes. The simple truth is that many readers will stop reading your story if it's riddled with errors, even if the story is interesting.