In a word, like all aspects of the writing process, finding an agent takes WORK; it begins with research.
The easiest way to get an agent is to be referred by a friend who has one or to meet one at a publishing conference, like the annual event ACW presents. If you don’t happen to have a friend who has an agent, then some writers find one by word of mouth through their writing groups, workshops, graduate programs, or open mics, or by asking important questions and sharing information with one another. Publisher’s Marketplace is an industry newsletter that announces which books have been sold to which presses and which agent arranged the deal. Poets & Writers magazine also profiles agents periodically.
The best trick to finding an agent, however,  is to look for books similar to yours (similar in either content or style) – and read the acknowledgements page. Writers almost always thank their agents in their acknowledgements. If that agent was interested in their book, there is a good chance he or she will be interested in yours. This connection will also help you write a stronger query letter since you can compare your book to the book the agent already represents, proving you’ve done your research. Of course, you will need to show how your book is “the same but different” from the book already published.
From there, find out how to contact the agents. As with any query or submission, it is important to expressly follow guidelines to the letter when approaching agents. Send only and specifically what is asked for!
Another amazing resource to find an agent and research publishing, etc, is the Association of Authors’ Representatives, Inc. It hosts a database of agents, FAQ, newsletter, and links!