Know your characters. Write a full biography for each of the main characters. Define what the characters want and what is preventing them from getting it. Outline the characters' whole lives, not just the parts to be included in the screenplay.
Decide on a theme. Theme defines the point or purpose of your screenplay. Every scene should relate to this central purpose. Make your theme universal, meaning that it is relevant to a wide audience.
Know your story. Each 25 percent of the screenplay is a critical turning point in your story where the action goes in an exciting, surprising new direction. Decide what happens at these critical turning points. Outline your scenes and define how they relate to the theme. When you can tell yourself the story in full, you are ready to write. Write the critical scenes first.
Make it sellable. Assuming your end goal is to sell your screenplay, consider how to write it to make it attractive to buyers. Reduce as much as possible scenes that make the movie too expensive to make. These scenes include such things as multiple or exotic locations, lots of rain, period sets and costumes.
Compare with produced movies. If your idea is similar to other movies, make it more unique.
Write for a collaborative medium. A screenplay is a recipe or a blueprint for a movie. It must be written in an industry-standard format. This format informs the planning of other participants in the movie-making. For example, the header for each scene states whether the shot is interior or exterior, where the scene is located and whether it is day or night. Both the interior/exterior and the day/night designations tell the lighting crew what kind and how many lights may be needed. Part of writing in the proper format includes not writing for other people's jobs. For example, never write camera shots into a first draft. It's also not necessary to specify what characters wear unless it is critical to plot.
Bring a critical eye to your own work. When you have a first draft, look for common beginner problems. There is not enough conflict. The main character is not active in the story. You've glossed over the emotional scenes. Characters are talking heads that move from one location to another without any real action.
Challenge yourself to write it better. Only send a screenplay to producers, agents or festivals when it is really ready. After you complete a draft, set it aside for a couple weeks, then see if you can improve it. Rewrite. When you think it's the best you can make it, get feedback from screenwriter groups or script analysts and rewrite it again.