Pick a plethora of novels, short stories or biographical sketches to read. Try to read them in a short of a sitting as possible; it will help you conceptualize a filmic version of the stories.
Pay special attention to themes in the story that are popular themes for films; themes like love, coming of age, thriller, comic misunderstandings and quests are especially common. Write down some of the themes that you see throughout the story.
Make certain that your story's protagonist is interesting enough to have a whole movie constructed around them. Think of your favorite films' main characters and compare the character of your story with that person; if you are pleased then you may have a good main character. Recognize if a viable conflict exists in the story and if it can be translated visually into a film.
Ask yourself whether this story takes you to a new and interesting place. Decide if it is a niche that more than one person can sink their teeth into. Also, more importantly recognize if this story would generate enough revenue for a studio to produce. Identify the hero's journey in the story, since it is a good way to outline a screenplay of any genre.
Develop a rating system to help you identify good screenplay story elements and a way to compare different stories you have read. Rate each story you read, bearing in mind different elements of: relatability, production value, theme, character development, plot and genre fit.