Write a one-sentence summary of the story and take as much time as you need. This sentence helps you throughout the writing process by reminding you about the most important component of your story.
Develop your most important characters. Before you write the story, write about the people in your story -- a lot. For example, you could write one-pagers about what each character does on a day-to-day basis or a journal entry from the perspective of a character. All of this writing helps you understand how the character will react to circumstances in the story.
Detail the situation that the story is about. This is what causes the story to happen, which you described briefly when you wrote your one-sentence summary.
Spend the most amount of time on the first paragraph because it sets the stage for the story (Reference 2). It must make the reader curious. Give the reader enough information to know what's going on, but not enough to spoil the story.
Finish the story with a resolution. This resolution doesn't need to be a happy ending, but it must be an ending. For example, if your character chases after an object, then he either needs to find it, discover that someone beat him to it or discover that the object never existed. This gives an ending/resolution to the story.