I'm in NI so our rules might be a bit different to that of GB, but I'll try to help.
As you're over 16 you can join the adult division. You could also join a Quad. Division (Caters for children and adults) but I would recommend a full adult division.
I'm not sure whether any existing first aid qualifications will count. You very may well have to go through the full training again for insurance reasons.
You'll work your way up through the training ranks and will eventually be allowed to drive the ambulances if your division is anything like ours! The training rank structure is below, normally each rank will last a minimum of 6 months, this is to make sure that people don't come into SJA to get a free FA qualification and then leave.
TFA (Trainee First Aid) ->
AED(Auto. External Defib.) / FA(First Aid) ->
AFA (Advanced First Aid) /CRU (Cyclist Response Training is option then)->
PTA(Patient Transport Attendant (You'll be 18 at this stage and should be allowed to drive as the volunteer equivalent of PCS (Patient Care Services) ->
ETA(Emergency Transport Attendant) ->
Response Driving (ROSPA Gold) ->
Blue Lights Driving Course
In my experience it's good fun and the people are great banter.