Most publishers and agents ask for a synopsis when you submit. If you need a proposal, the synopsis is almost always a part of that proposal. I recommend you write a three page synopsis and one page synopsis. The requirements for submissions vary, so if you have both lengths on hand you should be all right. The thing to remember about a synopsis is it will be the worst writing you’ve ever done.
Here’s some things that might help you write your synopsis.
A synopsis is the summary of a full length novel. This means you have to tell the story in as few words as possible. This is the time for you to tell, not show.
The first paragraph of your synopsis should be a short blurp about the novel. Think about the paragraph you would write for the back cover of your book to catch people’s attention.
Single space a synopsis, and have one inch margins and size 12 font.
Write the synopsis in present tense.
Tell the main plot and introduce the main characters. Don’t worry about subplots or minor characters here.\
Tell your ending. You don’t want to hide your surprise ending here. You want to show it off and let the people your submitting to know the novel reaches a satisfactory ending. If you don’t convince them of that, they might not be interested in reading the whole manuscript.
Here’s some links that will help you write your synopsis.