Write your novel synopsis in the present tense. Your goal is to tell the same story as in the novel, without giving away elements that should be a surprise in the novel. Writing in present tense isn't mandatory, but it's the most common format for a novel synopsis.
Structure your synopsis like a mini version of the book, adhering to the beginning, middle and end method of story telling. Include limited dialogue and focus on strong action elements. Any dialogue you include in the synopsis should be dialogue that is crucial to making a plot point or showing something about a character.
Keep the paragraphs in your synopsis small. The idea is to create a sense of urgency. You want to move the editor or agent through the story at a pace that makes them immediately want to pick up the manuscript and read it through.
Limit the time you spend describing characters in your synopsis. The place for character back story is in the novel itself. Use the synopsis to tell your editor or agent what the characters want and how they go about achieving their goals in the story. Keep physical, emotional descriptions down to a one or two sentences that lead into each character's place in your novel.
Describe the novel's ending. Don't try to be coy and secretive with an editor or agent. Leave that for the teaser on the back of the book, aimed at readers. An editor or agent needs to know how the novel ends. They need to know what they're going to publish. Tell them the entire story.