So where to start? Here are a few handy tips:
Start with the basics
There are a few things that should always be included. A title (if it's a working title, add a note to let the reader know), an approximate word count and an indication of which genre or audience you're aiming for.
Introduce your main character
You may have more than one main character than needs a mention, but try and limit your cast. Your synopsis needs to be easy to follow and if you add in too many people, readers are likely to grow confused. It's ok to miss out even quite significant characters here.
Highlight their main motivation/peril
To drive a reader forward, there needs to be some sort of question, crisis or premise that the story promises to solve in one way or another. This should be front and centre in your synopsis.
Keep things simple
Boiling a whole story down into something short and snappy is tricky and the best way to do it is to take as much of a bird's eye view as possible. What are the biggest plot points? Who is your absolute main character? If you had to pick one theme as the most important, which would it be? Include only the biggest and most prominant.
Tell the whole story
It's tempting to end a synopsis on a cliffhanger or to be coy about resolutions. Don't! A synopsis is there to show an agent or publisher that you know exactly where your book is going. If you leave out the ending, they might think you don't know what it is yet. A synopsis should break down all of the most major plot points, including the ending.
Take the time to craft it carefully
Your synopsis is really another sample of your writing and you should approach it with as much attention as you give your novel.
Check the guidelines
Different people and agencies will have different requirements and you don't want to send a few hundred words to someone who is looking for several pages, and vice versa. Following individual guidelines will give your synopsis the best chance.