(Re-running this entry from a couple years ago. I get a lot of questions about this subject. Hope it's a start.)
So you want to be a published author? You've written your novel first. You've workshopped it with writers groups, classes, and others who have an eye for story, grammar, and commercial appeal. Then... and only then... are you ready to shop your work out to an agent. A literary agent.
This is a person who, upon looking at your query letter and maybe the first five pages of your manuscript, will decide whether or not to read your entire manuscript. Once they read it, they will decide whether they are enthusiastic enough to represent it... enthusiastic enough to sell it to a publisher... enthusiastic enough to stick with you through rounds of rejection until the magic moment when you get an editor at a publishing house willing to take a chance on your manuscript.
There are good agents and bad agents. Agents with good reputations and bad. Agents with strong clout with publishers. Agents still establishing clout. It's good to do research when you're at the agent-hunting stage.
And one more thing... you NEVER have to pay an agent. Not for reading your manuscript. Not for calls. Not for postage. Not for (for you wannabe models) photos. Agents get paid only when they sell your work... and they get paid AFTER that happens. Usually it's a 15-20% cut of what your work earned in the sale... and that amount for the rest of the book's life -- the book they sell.
My agent is great. Smart. Funny. Around the same age. Very ambitious. Confident. And what made me go with him compared with others who were considering me at the time was that our conversations about my work and my goals in publishing were comfortable. I felt like I was talking with a brother/cousin/friend with business savvy. And he was enthusiastic about Down For Whatever (though at the time, it was originally titled Same Script, Different Cast). One agent, who is one of the most sought after -- a lot of people want this company to agent them -- talked to me like I was in third grade. That was weird. I found my agent because I had finished reading A Love Noire by Erica Simone Turnipseed, and since I liked the book, wanted to see who represented it. That's how I found Nicholas.
So... for homework. Take a look at some of your favorite books. See who is the agent. Most writers thank their agents in the acknowledgements. Start making a list. When you're ready to agent shop, you'll be ready.