Understand the three primary elements of a movie script. There's the scene heading, the action, and the dialogue. These elements are the bare bones for presenting your story in screenplay format. Format the page margins of your screenplay to 1 1/2 inches on the left and 1 inch everywhere else. These margins can vary slightly, but the left should always remain at 1 1/2 inches to leave room to bind the script.
Write scene headings as INT. or EXT., and include the location of the scene as well as the time of day (indicated as DAY or NIGHT). Your proper scene heading will look like this: INT. OFFICE--NIGHT. Use the same margin settings as the page margins for a scene heading.
Indicate action in your screenplay in standard upper- and lower-case sentences with margins set the same as the page margins. Separate paragraphs with a single line and be sure to write all action in present tense, describing what takes place at the moment it takes place. Keep it simple, and don't rely heavily on adjectives and adverbs.
Proceed dialogue with the name of the speaking character. Write the character name in all capitals beginning at 3.7 inches on the left margin. Write dialogue in upper case/lower case with the left margin beginning at 2.5 inches. Dialogue is important to character development in a screenplay. Pay attention to the dialogue you write, and make sure that each character has an individual style of speaking and that it remains consistent throughout the script.
Judge the length of the screenplay by estimating about 1 minute of screen time per screenplay page. Structure your story in three acts. The inciting incident would take up the first 30 pages, the main body of the story (the struggle or quest) will be about 60 pages, and the final 30 pages will wrap the story up. This is not strict, but it's a good guideline to go by. A majority of the films made fall into this structure.
Use screenwriting software. Screenwriting software handles the mundane formatting necessary for a script for a film and leaves you free to concentrate on writing the story. Movie Magic Screenwriter and Final Draft are big names in screenwriting software, but they can be a little pricey. You can use a free screenwriting program called Celtx (see Resources below) that does the same things as the big-name software, or you can use a free online service (see Resources below) to write your movie script.