You've got an idea, and you want to turn it into a story. How do you begin? First, determine if it is a story -- if the idea is big enough to carry a reader for a thousand words or more. If it has a beginning, a middle, and an end. If it's interesting. How do you do that? You outline. Either in your head or on paper, map your idea out. This doesn't have to be a formal outline, or even a very detailed map, here while you're thinking and planning. You're the only one who will ever see it. But like most everything else in life, the best results come from planning, so take the time to outline in some form.
So what do you think about while outlining? Decide how you will start your story. Decide what events will take place in the story. Yes, events. A story isn't static. Think of a fiction story as a race -- the characters start at the beginning and move toward a finish. If they don't move -- emotionally or physically -- then you've got a vignette, not a story. Conversely, if the characters wander all over the place without keeping an eye on that finish, you've got a travelogue -- or a story that lacks focus. If you've got a message that you feel is more important than moving characters from beginning to end, then maybe you should be writing an essay, not a story. Remember, fiction entertains. Even when you're conveying a message, you've got to make it interesting.
What else do you think about at this stage? Well, in what genre will you write your story? What is "genre"? It's the category your story will fit into -- romance, mystery, Western, science fiction, fantasy, and so on. If you read a particular genre, then you might naturally choose to write your story in that genre, because you will already be familiar with its conventions.
You don't have to fit your story into a genre, of course, but if your idea revolves around a puzzle that needs to be solved, you might write it as a mystery; if it focuses on two people falling in love, then you've got a romance; if it involves things that don't really exist (yet) in our world, then your story could fall into the genres of science fiction or fantasy. Deciding what category your story will fall into helps you focus the story you're preparing to write.
Okay, what else? Characters! Stories are about people -- or creatures similar enough to people that readers can identify with them. You have to have at least one character, the person whom the reader will "meet" at the beginning of the story and the person who will move through the story events to the ending. This character is called your protagonist. He or she or it has to seem like a real person so readers will care about what happens to the protagonist and want to follow him or her through the story to find out.
How do you create a realistic character? Again, with a little planning. You have to have a good idea of your character's personality so you can make how he or she reacts to events in the story seem realistic to readers. Think about how you act in a given situation, and think about how your friend or your child or your father would act. Differently, right? Your characters will react to events in the story based on their personality and background. Developing a personality and background for your character is called a character sketch. Since characters are the story, it's important to decide on the details of their personalities.
Next, decide where (and when) your story will take place. In the Old West? On a distant planet, in the far future? In Minnetonka, last summer? This is your story's setting. It helps shape the story (would you find cattle rustlers in New York City? I think not!) and even how the characters act (women in the 1700s had fewer freedoms and more strictly-defined roles than contemporary women, for example). It also paints that big, gray canvas in the reader's mind with details that make the story seem real.
While it's not vitally important that you know before starting to write, it doesn't hurt to think about how long the story should be. Look at the outline you created for your story. Are there a lot of events between the beginning and the end? Are all of those events necessary to get the protagonist from the beginning to the end? If there are too many necessary events to fit comfortably into a few-thousand-word short story, you might have a novella (a story between 20,000 and 50,000 words) or a novel (a story over 50,000 words) in the making. But if there is just enough material for a short story, don't pad it out with extraneous events just to make it a novel. Readers don't want to take the long way around to the end when the direct route is more effective.
You've got the rudiments of a plot (that's what you did when you were outlining), you've got characters, and you've got a background for the story's events -- a setting. Guess what? You're ready to put hands to keyboard or pen to paper, and ... write!
And if you're afraid to start, remember this: nothing's carved in stone. It doesn't have to be perfect, the first time around. Just get it down on paper. After all, it's that final act that defines a writer.