I would suggest visiting backstage.com for a directory of casting agents and managers. Not all agents will want to represent you though and you want to choose an agent that will meet your needs. This means you need to do some research to find the right agent. Then you need a good set of headshots and examples of some work you have done to showcase your talents.
Another very important step in the process is to network, network, network. Find other people who have agents, either friends or friends of friends or friends of friends of friends, and ask to be introduced or arrange to meet them at an event. This will give you a good start.