One for you, one for them. Many writers find writing with a synopsis too confining, they are 'seat of the pants writers' and need to get into the book before they can know where it is going. Others cannot function without a synopsis. Once you are published, you'll be allowed to submit a partial, which means you don't have to write the entire book before you can submit. In addition to the first three chapters, you'll need a detailed synopsis, so learning to write a good synopsis is imperative.
Some authors like to start their synopsis with a teaser line.
One line that capsules the entire plot line. Time and setting should be included. Many synopses are bogged down with too much information. The editor doesn't care if your heroine's hair is red, or if she wears a nice perfume, she wants to know the meaty stuff. The story section is always written in present tense.