Identify your conflict. Without a conflict, there is no story to tell. Two of your characters (at least) want different things and only one can get what he or she wants.
Think of a triangle. Your story should take the shape of a lopsided triangle. It starts on the left corner, which is your first paragraph; it progresses in a long, sloping line to the apex of the triangle, which is the climax of the story; it then falls quickly to the right corner, which is the last paragraph of your story. This will give you an idea of how many plot points you need before the climax and how many afterward.
Start with an unhappy main character. If your protagonist is happy, why would he or she embark on a struggle that will take him or her to the climax of the story?
Give the main character obstacles and encouragement. It must be difficult but not impossible to obtain the goal. Remember that other characters are terrific sources of both obstacles and encouragement.
Put the gun on the mantle. A writing aphorism states that if one character will shoot another at the end of a play, the audience needs to see the gun on the mantle at the beginning of the play. Fill in your story with the details needed to make the climax inevitable.
Make the reader nervous. Just before the climax of the story, it must seem that all is lost for the main character before some last-minute action turns things around and brings on the climax of the story.
Wrap things up. After the climax, make sure there are no loose ends, no unresolved plot points that would leave the reader unsatisfied.