Technician assistants usually learn on the job. The job can be competitive and you may need to start as a receptionist or kennel assistant and work your way up.
Usually they require atleast one years experience in the industry. As an ACA, over time you can apply for their vet tech internship if you're lucky enough to be chosen.
Depending where you are, a diploma from an accredited college is either an asset or required. (Canada) There are many colleges out there, some even specialize in Veterinary Assistance.