Coolant fan switch

My vehicle recently overheated. Pressure built up and the radiator hose snapped off the radiator itself, taking part of the engine inlet that it was connected to with it. I have replaced the radiator but my coolant fans will not run. I know that it has to be the coolant fan switch. However, when I look online I see several different places where these switches go: in the water outlet, on the thermostat housing, and on the radiator itself. There are also parts claiming to be "upper" and "lower". Now since I have only three hours of research/experience in working on cars, I am a little clueless as to which product I need to buy to get the job done. Does the positioning of the sensor particularly matter between those three spots and if so, which one should I attach the sensor to, to activate the fans? Do I need more than just one sensor? What does "upper" and "lower" mean? Lastly what would be an affordable, part/brand that works well, to order for my particular make and model?