An agent is more interested in a career writer, not a one-time deal. Sending out unsolicited material is not recommended. Send a query letter with a self addressed stamped envelope (SASE). Introduce yourself, your story, and your career goals. You may even include a few sample pages. Don't include a treatment or a detailed synopsis. Some will tell you otherwise on this, but the chances of getting the script in their hands go down the more story details you reveal up front - I mean, who wants to know the ending of a movie before they've seen it? Market it to them, pitch it to them in your query letter. If they're interested in reading your script, they'll tell you. Most of what you'll receive are rejection letters. And those who read your script will want to know what else you have since the chances of a first-time writer with a first script getting an agent is slim to none (been there, done that).
Before sending your query letter, you might even consider a cold call. I did this with every agency I queried. Secretaries and receptionists are the gate keepers and are your best friends when trying to get your foot in the door. Sending a letter "to whom it may concern" is bad form. Talk to a receptionist (or whoever happens to answer the phone), ask if they're accepting new material from new writers, and ask for a name. Address that person by name in your query. If you do happen to get an agent on the phone (rare, but it did happen to me a few times), be polite, very respectful, and be sure to thank him or her for their time both on the phone and again in your query letter.
I ran this gauntlet several years ago and learned mostly what not to do, but I did get my script read by several agencies and did get some positive responses to my writing style. Of course, they passed on the story, but it opened my eyes and my ambition. Make certain you study your craft intensely and have more than one script ready before approaching an agent. I won't make this mistake again.