I agree with EOD Guy in that you need a relay. If you don't install a relay, you will be replacing on/off switches on a pretty regular basis. A relay is probably less than five bucks at the local auto parts store, so it is not like they are expensive.
A temperature sensor is a great idea as well, as that will allow you to forget about the fan and let the temperature of the engine control the operation of the fan as required. With or without a relay, if you simply use an on/off switch, you will need to remember to turn the fan on to prevent a hot engine, and then remember to turn it off to avoid a dead battery.
Dennis W. Parks
Author of automotive how-to books