Before you commit to weeks and months of writing a screenplay, write a logline.
Sketch out the story of the story in twenty-five words or less. (Sample: When a killer shark unleashes chaos on a beach community, a local sheriff, a marine biologist, and an old seafarer must hunt the beast down before it kills again.)
Write the logline before the script. When you start with a logline, you’re helping yourself identify the core of your story, making it easier to focus on the story lens of your screenplay.
Less frustration and hair pulling. More words on the page heading in the right direction.
Bonus points if you can wangle a theme in there.