Devise a good, original story that can be pitched in a sentence or two, and keep in mind that high-concept does not have to equal "big budget." Writing a script that can be filmed at a modest budget opens it up to a greater number of potential buyers.
Learn about the process and craft of screenwriting. Read books like Robert McKee's "Story" and Lew Hunter's "Screenwriting 434" and focus on the elements that help make a script great, including proper three-act structure, character arcs and dialogue.
Purchase an industry-accepted screenwriting program such as Final Draft or Movie Magic Screenwriter. These programs will properly format your script as you write it and save it in a file format widely used by agencies and studios, which is important for when you send it out as an email attachment.
Map out and use note cards to outline the major plot points and scenes from beginning to end. Knowing where your story is going will make it easier for you to write the actual script.
Write the screenplay, keeping descriptive paragraphs short and to the point, and remembering the basic tenet that movies are supposed to show rather than tell. Read the script when finished and be prepared to rewrite some or all of it at least once.
Register your completed screenplay with the Writer's Guild. This will establish a copyright and ownership of the material, which is vital in case of plagiarism or theft of your work.
Show the script to peers or anyone you trust to give an honest opinion, and take their suggestions into consideration for rewrites. Join online writers' workshops, like the one at Francis Ford Coppola's Zoetrope Studios, to read others' scripts and get educated feedback on yours.
Submit the script, when you feel it is polished and ready, to screenwriting contests sponsored and judged by industry professionals. This can be a great way to get it read and noticed, and the winners may also achieve a sale and/or representation as a result.
Obtain a list of agencies that accept submissions from new and unsigned writers. Follow the protocols each agency sets for those submissions and inquiries (i.e. no phone calls, no unsolicited email attachments). One such list is available at Scriptologist.com.
Use any connections you may have to get your script in the hands of anyone in a position to get a deal done. With all the screenplays written each year, the biggest factor in getting one sold can still boil down to knowing the right people.