Understand the strict industry-standard script format. This includes writing everything in 12-point courier font, exact margin settings for the various elements, such as dialogue, character names, scene headings, general direction and personal direction, and additional elements, such as writing sound cues in all caps. See Resources for specific guidelines.
Pick a formatting program. Two popular programs are Final Draft and Movie Magic (see Resources).There are also freeware screen-writing programs available for download, but these are usually of questionable quality.
Sketch a rough outline of events in progressive order for the script. One page of screenplay is equal to one minute of movie. An average length for a first-time screenplay should be from 90 to 100 pages. Scripts are typically broken into three acts. The first act should familiarize the audience with the main characters and introduce the conflict. The second act should expand the conflict and will include the bulk of the story. The third act should resolve the conflict.
Focus on characters. Characters must grow and experience conflicting motivations to interest the audience. Character dialogue must sound natural and distinct. Practice saying the dialogue aloud as you write.
Write the first draft. The writing process can take anywhere from two weeks to several months, depending on your experience and the amount of time you devote to writing.
Ask for feedback after the draft is complete. Some writers join writing circles, while others get feedback from friends. You might also consider an online writing circle, such as http://www.critters.org or http://screenwriter.gather.com.
Register your completed script for copyright protection. This can be done through the Writers Guild of America (see Resources). A more formal and costly option is to register a script with the U.S. copyright office (http://www.copyright.gov).