Read several movie pitches, available online. It is often said that a pitch (a short, two-page synopsis) can sell or sink a much longer and more polished script, which might be as much as 150 pages.
Create a strategy for pitching your movie. As Bob Kosberg points out, “not every great movie is pitchable.” Kosberg says that high-concept movies, which sound funny in a sentence or two, are much easier to pitch and sell than some movie ideas that are solid but difficult to pitch. He compares “Shallow Hal” (which is pitchable) with “The Big Chill” (which would be difficult to pitch). Either your movie idea will seem like a great pitch immediately, or you might need to send in a finished script.
Develop your pitch. The pitch should remain brief, but should highlight the novelty of the idea, as well as the development of characters. You should stay away from casting the movie yourself. Remember that, for every hundred movie pitches that are optioned by movie studios, only one eventually becomes a movie.
Send your screenplay to prestigious, annual screenwriting contests, if you don't get the attention of production companies or agents. One of the best known is the Nicholl Screenwriting Fellowship (the link is below, in Resources). Winning or being a finalist in one of these competitions can help bring some attention from producers or agents in Hollywood, looking for ideas.
Send query letters to agents or production companies, giving a short bio of yourself and a few lines summing up your movie idea. When you select agents and production companies, you should research on their websites the type of clients they have or films they make. Both large and small production companies are searching for new screenplays. Sending a big-budget movie to an indie company would be a waste of time, but you should cast as wide a net as possible, submitting your screenplay to as many production companies as possible.
Some screenwriters make a good living without having an agent, but the contacts that agents have are usually very valuable and can help secure a more profitable contract.
Begin writing the script, even if you don’t get the interest from an agent or a production company. Contemporary movie scripts do not describe camera angles, and stage directions can be written in vernacular. When writing a movie script, the dialogue and the narrative should be well-polished and you should stay away from aspects that might turn actors or directors away. For instance, describing camera angles might turn away directors, and listing character expressions might turn away actors.
Register your screenplay with a writers guild, to protect your work. Though copyright resides with a writer automatically, doing this can give you credibility if others try to steal your work. The Writers Guild of America, West, is the labor union of screenwriters and represents writers when authorship and payment is contested. The guild also provides a method for buying back the rights to your screenplay, if it goes un-produced for over five years.