According to this guy this is what you can do:
“Find out who the agents are for writers in your genre and then scan the web for their home page. Are they accepting new clients? If so, submit the first three chapters, a brief (1 or 2 page) synopsis and a short cover letter asking them to represent you. If you don’t know how to write a synopsis, here are 99 of them, critiqued and commented on by a real agent.
In the cover letter you state your previous publication credits, if any, and also what other work you have in the pipeline. Include a stamped, self-addressed envelope for their reply. That’s the general idea, but you should check their guidelines for more.
One of Miss Snark’s regular readers posted a comment detailing her search for an agent here. Excellent advice within.
See my article on rejection for more on this topic.
The bad news is that agents are just as flooded with manuscripts as editors, and therefore they have to be just as picky. Don’t be surprised if you have trouble at this stage. A lot of trouble. The good news is that there are more agents than publishing houses, and once you get an agent they can make a better approach with your manuscript than you can. If you burn up all the publishers with your manuscript, what are you going to do next?
Think of getting an agent as a step or two up the ladder of publication… an agent will only sign you up if they believe you have something they can sell. And they’re in a better position to sell it than you are.”
This person gives another but similar take on the process.
Neil Gaiman (one of my favorite writers) gives some useful points from people he knows who are in publishing.
The first thing to do, of course, is to start writing. Next you might consider getting shorter pieces into publication, such as short stories sold to magazines, so you can show you can meet deadlines, tolerate being edited, and can write something at least some people want to read.