Outline the general structure of your story. Do this by writing three headings on paper or using a word processor. Those headings should be labeled Act One, Act Two and Act three. You can have more acts, but the three-act structure is the most common and typically forms itself naturally in a well-written story. Breaking your story into a three-act structure can help you focus. Write these headings spaced far enough apart to give yourself room to write beats and scenes.
Within each act, begin writing the "beats" of your story. A beat is the smallest bit of information that can be measured in a story. It can be any piece of action that has an effect on your story. Limit the beats you write. A beat looks like this:
1. Jane answers the phone.
The above is a beat, but depending upon the story, it may or may not be one you want to include. Unless Jane answering the phone plays an important role for a story element that follows, this isn't a beat you want to include. A beat you might include is "Jane finds Tommy's gun" or "Jane purchases a gun." These two bits of action will probably (and should) figure into the story later.
Continue to write these small beats, adding and removing them until you feel you have a structure for a sequence. After each set of beats that comprises a scene, write in capital letters SCENE and number it. Several beats make up a scene. When you have two or three scenes that comprise a major sequence of events in your story, label these scenes as a SEQUENCE and number them.
Write beats, scenes and sequences until you have your first act. The beats, scenes and sequences that comprise your first act are written in beneath your Act One heading. These story elements, by the end of act one, should bring you to an inciting incident. The inciting incident is the event that sends your main character on the journey to achieve his goal.
Structure the final two acts on your story sheet the same way. The beats in act two should set up the opposition for your main character. Again, choose the beats you include wisely. Beats in act two will be events that make it difficult for your main character to succeed, culminating in a sequence that puts your main character at his lowest point. Beats in act three will comprise events that eventually lead your main character to overcome and achieve his goal.