First basic check you want to do is start the engine, bonnet raised ,switch aircon on..Your engine cooling fan should start running at half speed ,then it will switch off ,then on and off etc ,If it doesn't the resister in the top of the fan cowling will probably have failed ..When you switch the aircon on is the pump definitely working ,and do you definitely have gas in the system ,as a vehicle will lose it's gas over a period of time and you may just need a simple re gas