To be a good screenwriter, you have to know what makes a good story. The foundation is understanding what makes a satisfying story. It is all about a character the viewer/reader can identify with, and the goal or motivation that the viewer/reader can adopt. It is all about the character growing and doing something. You absolutely must read the following:
The Writers Journey: Mythic Structure for Writers, 3rd Edition by Christopher Vogler
Story: Substance, Structure, Style and the Principles of Screenwriting by Robert McKee
Screenplay: The Foundations of Screenwriting by Syd Field
Save The Cat! The Last Book on Screenwriting You'll Ever Need by Blake Snyder
These get you started on what you must know. Some will say these resources claim that there is a formula for good story and that a true artist must never be bound by a formula. The most convincing argument against this garbage idea is a comparison of Star Wars and Rogue One. In Staw Wars, a likeable protagonist is drawn into a comic battle and has to have a character arc of development AND DO SOMETHING to move the story along to finally overcome in the end. Very satisfying. In Rogue One, the protagonist does absolutely nothing to further the story and is the most passive protagonist in the history of story. Everything happens to her but she doesn't do anything to move the story along. No development, no action, no good. Read these books and learn what good story is about. You also have to know good dialogue. You have to know people and characters or your stories will be about cardboard characters. Learn about these things and you will be far ahead of most people writing screen plays.