Decide on the screenwriting software you want to use. A screenplay, unlike a novel, needs to be written to specific formatting guidelines. Screenwriting software does the formatting for you, leaving you free to tell your story. The two big screenwriting programs are Final Draft and Movie Magic Screenwriter. If you want something cheaper, Celtx is full-featured screenwriting software available free, and Scripped is a free web-based screenwriting program (see Resources). Both of these scriptwriting programs automatically format industry-standard scripts and include tools for developing your story.
Write your story in spec script format. Spec stands for speculation. It's the kind of script a writer uses to break into the scriptwriting business, whether through the sale of that initial script or through an offer by a producer to write another script. A spec script differs from a shooting script. Don't include scene numbering or too many shot calls in your spec script. Leave those things to the director. You want to tell a straightforward story just as you would in traditional prose writing, except it will be formatted as a screenplay.
Use only action and dialogue in your script. A screenplay is what can be seen and heard on the movie screen, written in the present tense, as if the action is happening now. Use scene headings, formatted like this: INT. HOUSE - NIGHT. This tell us the scene is an interior (use EXT. for exterior), taking place in a house at night. All action should be written in upper- and lowercase. A character name, in all capitals, precedes character dialogue, which you write in upper- and lowercase. Keep a screenwriting format guide handy for specifics on formatting (see Resources).
Register your script with the Writer's Guild (WGA) and submit your finished script to producers and agents using an online pitch service such as Virtual Pitchfest or by consulting an online industry contacts database (see Resources). These market databases give full contact information, the types of scripts each contact is interested in and full submission guidelines. Follow the submission guidelines of each contact to ensure your script gets read. If the guidelines ask for a synopsis of your script, write a one- to five-page synopsis and send that, along with a short query letter stating who you are and listing any previous film credits you have. Never send your full script unless it's requested.
Follow up after three months. Chances are good your script isn't being considered if you haven't heard anything, but a follow-up by email or postal mail is acceptable. Make your follow-up letter short. Remind the producer or agent who you are and ask about the status of your screenplay. Provide as much contact information as you can in the letter. While you wait, send another script out.