Put yourself in the reader's shoes. Remember those time when you were bored and scourged through tens or hundreds of novels, hoping to find an interesting one.
-it could be a genre you like
-it could be an interesting concept you've never came acrossed
-it could be two or more genres you like and imagined both of them crossed together in a novel, and then a novel that fulfills it or most probably close to it appears
Easiest one would be to open up your Novel Library and look at how your favorite authors/translator put up their synopsis
Hope that helps buddy. Maybe you can check out how I did my novel's sysnopsis
Surviving Apocalypse with Powers of Fictional Characters