Somebody more up-to-date is bound to reply.. but just in case not, I believe an A/G operator requires no qualifications and training would probably involve sitting with one of A/G ops until they are happy for you to work alone. The next stage up is AFISO, which does involve certification, although is is not an Air Traffic Control service. Try a search on here for "AFISO"..
I would advise you to speak with the people at your local airfield and seek advice from them.
Lastly, it must be said that a lot of A/G operators are licenced ATCOs doing the jobs in their spare time..