Get out a piece of blank paper and draw three large boxes on it. Label them Beginning, Middle and End. It doesn't matter whether the boxes are the same size, as long as you leave enough room to write a few sentences inside them.
Write down what happens in the beginning of your story in the first box, as simply as possible. If you can condense it down to one sentence, that's ideal. It should say what the main character is trying to accomplish in the story. Don't worry about your exact choice of words.
Skip to the last box, ignoring the middle box for a moment. In one or two sentences, write out how the story ends. Address the following: Does the main character achieve his goal? How does the character do it? Don't try to tell the entire story, just the very last part, the decisive action at the end of the story that leads to the character achieving the goal (or not).
List all of the things that could keep the hero from achieving that goal in the middle box. It could be many things: fear, conflicting desires, an enemy working at cross purposes, or a mysterious message. Brainstorm a few ideas and jot down your favorite ones.
Create new boxes to expand your story. Divide another piece of paper into several boxes. Write each of the obstacles from your "Middle" box in its own separate box. Then, beneath that, describe how the character overcomes that obstacle. Draw as many boxes as you need for all of the middle obstacles. If you'd prefer to use index cards instead of drawing boxes, feel free. The point is to get your creativity working any way you can.
Take another piece of paper and draw two or three boxes. In the first box, write the sentence from your Beginning box. In each box after that, write down each step of the story needed to connect it to the first obstacle. Add more boxes if you need to.
Take one more piece of paper and draw two or three boxes. In the last box, write the ending of your story. In each box before it, write the minimum number of steps to link the last obstacle to the ending.
Put your papers in order: beginning boxes, middle boxes and end boxes.
Using your boxes as an outline, write your manuscript. Write one scene or chapter based on each box. If you have set this up properly, the entire story should be clearly laid out before you, from beginning to end. All you have to do is write it!