Most start out as pet dog obedience trainers and animal behaviorist's then specialize in service dog skill.
Volunteering at a shelter may provide good references and connections, volunteering at as a puppy walker or puppy raiser for a established service dog group will do the same. This is not a career for the faint of heart, it requires time, dedication, schooling and most of all a love for animals. People skills are also important.
This should get you started
So You Want To Be A Guide Dog Trainer?
Assistance Dog Trainer Profile
http://www.redgoldfish.co.uk/view_job_i ... sp?gid=328
Photo Credit: Baronet Photography
SERVICE DOG RESOURCES THROUGHOUT THE U.S. (BY STATE)
Service Dog Directory