when i start the car for some reason the Aircon comes on in drive which is most annoying when you don't what it on 

how can i disable this in the sharan .

Have you got it on the windscreen demist setting?

not sure its a standard car basic controls like the jetta on my golf which as the same controls if the heaters on the aircon stays off ,on the sharan the aircon comes on when the heater are on you have to switch it off manually its as controls on the rear seats for passengers i don't have a manual for it .   

If the heaters are set to windscreen demist it puts air con on aswell,try putting to feet position and turn air con off see if that makes a difference