Outline the setting of your story. If it takes place in the modern world, this should be pretty straightforward. If it takes place at some point in the past, then be prepared to do a lot of research to get the details right. If it takes place in an imaginary world, write down answers to the following questions: What is the political system like? What forms of technology do your characters rely on? What social struggles or wars are going on? What are the religious beliefs of the characters?
Write a sketch of your main character. This should be about one page in length. Describe her place in the world of your story, her personality, her goals and the central conflict that will engulf her in the course of the story.
Pen shorter sketches of the supporting characters. Chief among them should be an antagonist and any friends or family members of the protagonist who will play a major supporting role in the story.
Write a synopsis of the plot. Don't make it too detailed -- the details will be fleshed out in the story itself -- but make sure that you sketch in the beginning, middle and end. There should be one overriding conflict that your characters struggle with throughout, as well as several smaller conflicts that emerge from this one. If it helps, you can sketch it out with a graph.