Begin with the first sentence. It’s the most important. It should contain two elements. The first is the main character and what he/she was doing before the turning point of your story. For example:
Belva is an ordinary housewife living in Tulsa when …
Jack is reaching the end of an unremarkable career as a journalist when …
A young American couple honeymoons on a small island where …
The second part of this first sentence should introduce the nature of the turning point of your story, i.e., the event or incident that sets your story in motion.
Belva is an ordinary housewife living in Tulsa when she discovers her youngest son has the power to see into the future.
Jack is reaching the end of his career as a journalist when a young woman appears at his door with a story he can’t walk away from.
A young American couple honeymoons on a small island where dreams are promised and nightmares begin.
In a very short synopsis, or a simple slug line, your goal is to give the reader the central set-up of your story (who the main character is) and a brief glimpse of where it is headed (what’s about to happen to that character). The more you can convey the genre and the mood of the story, the more likely you are to find your target audience.