Now that you've assembled your list of literary agents, your next step is to eliminate the scam artists. Sadly to say, there's a bunch of them. The best resources are Preditors and Editors maintained by Another Realm magazine, Writer Beware maintained by the Science Fiction and Fantasy Writers of America and the Absolute Writers Watercooler. I would say that anyone that has earned the "Not Recommended" tag by Preditors and Editors or asks you for upfront reading fees shouldn't be given a shot at your material.
There are two theories about who to query first: Heavy hitter agents or hungry junior agents who work for established firms. My vote is for the hungry junior agents who are putting together their client list. You'll get more consideration and better service from them than from someone who's handling the current number one bestseller on the New York Times Bestseller list. To find these agents, check out which agents are attending the big writers' conferences and take a look at individual agency websites to see which junior agent represents your genre. Look at the literary agent forum at Writer's Net and the Absolute Writers Watercooler.
Once your list is assembled and prioritized, you can start sending out your query letter and if allowed by the agent's guidelines, sample chapters and a synopsis. You have done your query letter, haven't you? Writer's Net is great for trying out your hook and pitch on fellow writer's.
Not all agents are that easy to find. I kept seeing references to a shadow agent who I'll call Fiona Applegate (I'm protecting her identity just in case she bites on my query). She wasn't listed in Herman's book, the Guide to Literary Agents or any of the on-line directories. Google didn't have her, but I did discover that she had 20 or so clients and dealt almost exclusively by the phone. She would initiate contact with prospective clients by telephone rather than having them query her. I was intrigued and set out to discover hot to contact her. Working with several different esoteric search engines, I did find her agency and sent her a query letter. I'm patiently awaiting her call. Hope springs eternal.