Read a few screenplays of your favorite films so you can get familiar with the proper screenplay format. This will expose you to the structures and techniques used by screenwriters in the film industry.
Type and print your screenplay on standard white paper. Always use a size 12 Courier font. The margins should be similar to those of writing a standard letter.
Begin your screenplay with a scene heading which is also known as the slugline. This establishes the setting of your film and must be aligned with the left hand margin: EXT. A VAST EMPTY DESERT WASTELAND.
Refer to each character using all capital letters and place it 3.5 inches from the left hand margin: JOHNNY BOY, YOUNGEST SON, HANDSOME TEACHER.
Place dialogue below each character name and 2.5 inches from the left hand margin. Any words said by the character can run between 30 to 35 spaces to the right before returning back to 2.5 inches from the left hand margin.
Use parentheticals to give simple verbal directions that come before the character's dialogue. You will use this whenever you need to describe the way a character feels or does any sort of gesture or action.
Use the technique called an extension if you need a character to speak off screen or in a voice over sequence. The extension is placed one space after the character's name and is abbreviated. Use the abbreviation O.S. for dialogue coming off-screen from another character and V.O. for a voice-over narration in your story.
Use transitions such as CUT TO: or FADE IN: to end a scene or begin a new scene in your story.