Write a rough draft of the treatment. The treatment of a film is a detailed but brief outline of the story as a whole. List all of the major characters and when they'll appear in the story, the main twists and turns, and the full beginning, middle and end of the story.
Seek feedback on the story, compare it to existing films, novels and plays, and make sure you have a unique story. Rewrite the treatment, so that you have a clear outline of the film.
Break the treatment into a complete list of the scenes that are needed for the production. Whenever the setting of the dialogue needs to change, that constitutes a scene change. Repeat this for all scenes needed to complete the story.
Open your word processor or script-drafting software and start a new document.
Write the scene header in all capital letters. First, note whether the scene will take place indoors or outdoors, by starting with "INT" or "EXT" (interior or exterior). Next, write the location, and then add a hyphen surrounded by spaces ( - ). Finally, write the time of day during which the scene takes place. For example, "INT. COURTROOM - DAY".
Place characters in the scene. Write in stage directions, giving characters a location and activity to start the scene. For example, "ROBERT stands in the doorway, leaning against the doorframe. He looks in for a moment, then walks across to where JANET is sitting." Write as much is needed to establish the scene. If characters don't have dialogue, you don't need to name them in these directions.
Add in dialogue. Center the cursor on the page, then write the name of the speaking character in all capital letters. Carriage return to a new line, and add in the character's line. For example:
ROBERT
I have some terrible news, Janet.
Repeat writing dialogue and stage directions until you complete the scene. When finished, re-read the dialog aloud to make sure that it makes sense. Ask yourself whether the phrasing of the characters is natural, and if people would actually speak like that.
Start the next scene with a new scene header, and repeat until you've written all of the scenes. When complete, seek more feedback. Make changes based on the feedback.