They don't call it the dreaded synopsis for nothing. Many writers find it difficult to narrow the many facets of their story down to five pages. First, let me say, you will need to do some research to determine the length of your synopsis. Check the websites of the agents and editors you are targeting. I find five pages double-spaced to be a safe bet.
You will want to include the goal, motivation, and conflict of your characters and each of the main turning points. I've heard repeatedly that your writing voice should show in your synopsis, but I find that difficult to accomplish.
I usually write the synopsis after writing the manuscript, but my agent requested I write one for the third book in a series I had written yet. I went to bed asking the heavens for an idea. Not only did I wake up with one, I wrote the synopsis and it turned out to be the best idea in the series. I hope to repeat that event many times to come.
Don't worry, a synopsis is not set in stone. You can adjust it, and I often do, as you alter your story.