That's the easy part. The hard part is getting one of those agents to actually read your work once you do contact them and then (the real goal) getting one of them to represent your work and help you get it published. Each agent has their own submission guidelines and preferences, and it's important to pay attention to these. They pretty much all have one thing in common, though. They will all want to read a query letter before they will even consider reading your whole book.
If you don't know what a query letter is, there are lots of great sources out there that will give you the details of what is is, and what it is not. Basically, it's a very special, VERY SHORT sales pitch and hook for your book. You've got to pique their interest so they want to read more. To learn about query letters and get critiques on a query letter from a very helpful community, I use this website:
Many agents will also want to see a synopsis. The above website can help you with that as well.
I'll just take a quick minute to point out that, in the search for an agent, you should be a little wary of anything that seems too good to be true. Also, no real, reputable agent will charge you ANYTHING. They make a percentage (usually 10-15%) of the money that they get for you from a publisher. If they are asking you for money, run away.
Best of luck!!!