Before you begin writing, purchase or download scriptwriting software for your computer. Scripts must be written in a precise format with specific margins, headings and space requirements. Rather than do it manually through an entire script, it's much easier to use a program that will do the work for you. There are several options available, so research programs to see which one best fits your writing style.
Most screenplays consist of three acts. Act I introduces the characters, setting and plot. Act II begins when the main character or characters must confront a conflict or dilemma. Act II also calls for rising action or tension. Act III contains the resolution of the conflict and the story's conclusion. Once you know the three act format you can watch a movie and pick out the transitions from act to act.
When telling a story visually it's important to remember that the filmmakers will be adding the scenery, so keep the script tight by leaving out unnecessary descriptions. For example, you don't need to write "the moon casts a silvery glow over the landscape." Instead, keep it simple by writing, "Night. Moonlight." Also make sure that your props are important to your characters. The hero or heroine should't be carrying a gun unless it's vital to the story later on.
A professional screenwriting association can provide resources to help you with writing and selling your script. They offer writers' forums, blogs, tips, contests, links and a host of other services for screenwriters. Some prominent organizations include the American Screenwriters Association, the International Screenwriters Association, the Screenwriters Federation of America and the Writers Guild of America.