Your synopsis is a brief summary of your story. It should consist of a bird's eye view of the plot, as if from a mile above.
1) What is this story about?
2) Who are the main characters?
3) What do these characters want?
4) Why do they want it?
5) And what's going to stand in their way of getting it?
That's it! Notice this synopsis is general, and leaves plenty of room for surprise and suspense. The reader gets a sense of what to expect, without reading a lot of detail. That's all you need.
Concentrate on the key scenes that show the heart of your story. You have around six major parts to your synopsis:
1. Start out by encapsulating the plot to include the major conflict and the theme of the story.
2. Introduce the hero and heroine, their goals and motivations.
3. Include additional internal and external conflicts that impact on the main plot.
4. Include key scenes, plot twists and reversals that lead to the climax
5. Include the Climax and Dark Moment
6. Conclude with the Resolution.
The formula is always the same: a bird's eye view of the story; letting the reader know what to expect; withholding enough detail to save the suspense. Practice will make perfect for you.