The first thing you need to know—the thing that most people don't want to hear—is that for a fictional novel, you have to have your whole book written before you can even begin to query agents.
So, get your manuscript in great shape—go to writing classes, attend workshops, and get yourself into a writing group in order to get your book in fighting shape. But, really, if you think about it, that's the fun part. Then the real work begins:
You'll need to start with a reputable guidebook—I suggest Writer's Market in my Mediabistro class, but I’ve also heard that Jeff Herman’s guide is quite good, too. Read the section in the beginning about how to write a query letter and the etiquette of querying agents. All of that stuff is key—it’s just like applying for a job. Just because they are agents doesn’t mean you can be any less professional in your communications.
I'd also suggest using mediabistro.com to back up your book research. You need to be an Avant Guild member of mediabistro to access their content on "How To Pitch An Agent" content but in my experience, it was worth it.
Last but not least, you should also do internet research on each and every agent you plan to query. (Specific agent, that is, not merely the agency, although you should research the agency, too!)
Remember that you should never pay an agent to read or review your work. Agents get paid by selling your work and then taking 15% of the sale and royalties.
Allison Winn Scotch’s blog, Ask Allison, has *amazing* info on finding agents (she's also on myspace). Read the links starting from the oldest. Her advice is really spot on.
She also does a q and a, so you can email her if you have specific questions.
Allison also has a bunch of incredibly informative blog posts where she talks about her own personal experience, so check them out.
If you prefer your advice with a bit of attitude, check out Miss Snark. She no longer updates the blog, but the archives are invaluable.
And this blog post offers great insight on advances and how an author earns royalties.
I wish you the best of luck. Now, get back to writing!