1998 Plymouth Voyager over heating

We started overheating and ended up replacing the waterpump and thermostat and gasket for thermostat. Then we still overheated after about 15 miles of driving. Wewent to a mechanic that says it is some kind or relay cause the fan to cool the radiator does not come on unless we run the air conditioning or it could be temperature controll sensor. We are 2 disabled old ladies that dont know about cars and limited income. Can you replace a relay without being a mechanic ( is it in the fuse box under the hood) or does it need to be replaced by a mechanic and which one is it called? The car gauge for temperature of motor does not register when the car is overheating but does register when you turn car on. Also how much should I have to pay to replace and install a Temperature control sensor? And what does it do?