Anything Goes : Agent types by Shashu Hewett

Well... A manager and an agent do two, very different things so I suggest you first do some research and figure out which one best suits your current needs. The the way one goes about obtaining an agent or manager is to research agencies and managers and contact them by phone or email.

There are different types of agents and it depends on what your career path is. There are agents for everything from the actors and screenwriters you mention to below the line crew (key positions). Directors, etc. Also, if you are talking about for your production company or a specific project you can work with an agency to package your film. Your profile says author and screenwriter, so a literary agent is what you are looking for. As to the difference, SAG outlines the different from an actors position, but its pretty much a common definition. http://www.sagaftra.org/content/agents-and-managers