Make sure your screenplay is perfect. It is important that you read or have actors read your screenplay aloud so that it is in perfect shape.
Copyright your screenplay by mailing it to yourself. By doing this, you are protecting your ideas from plagiarism. Alternatively, you can register your script with the Writers Guild of America.
Write a query letter. Most agencies will not read a screenplay unless they are attracted to your idea. Write a short, one paragraph summary of your screenplay.
Go to the Writers Guild of America Website listed in the resources section and choose a few agencies to send your query to. The more, the better. Personally address each query letter.
Send out your query and wait for a response. Use this time to make sure your screenplay is in perfect shape and you are completely happy with it.