You will need to serve an apprenticeship to gain hands-on experience. You can expect to start out as a Camera Trainee before progressing to become a 2nd then 1st AC.
You could also work at a junior level in a film lab or camera equipment facilities house. However, since the core of the job is learning how to gauge focal length to such a degree that it becomes second nature, being around working cameras and learning how to use them is crucial.