Actually I was lucky.
I was at a point where I thought I would start looking for representation so I started searching a few rep sites and suddenly I got an email from an agent interested in representing me. I took it as a sign.
After many emails and questions and getting to know a bit one another I decided to sign with them. I know this is not the usual scenario. But if you are thinking of getting one I would look online. Children's Illustrators already has a section with agents so it's easier to just click and go to agent's websites.
Look and see if you like the artists they represent. If you do, try to search on boards (SCBWI) and the blue boards to see if someone has posted anything about them and you can ask around. If you find one that you like, try to see if you know any of the artists they represent and send them an email asking how it has been for them.
Then, I would read the agent's or rep's submission policies and go for it.