I suggest to research this site (if you haven’t already) a liitle better before posting.
There is plently of helpful tips and information that shouldn’t have to be spoon fed here. Try IMCA, which also give guidelines on requirements to become a ROV PT Trainee etc.
Start calling & knocking the doors of different ROV companies worldwide.
Make sure you have either mechanical or electrical background, both is better but none is bad!! Last thing we want out in the field is to train a new guy from step one, this is a wrench, get it? Good luck!! 8)