Well that problem is actually very easy to solve, you problem should be a bad coolant temperature sensor is the one that monitors the coolant temperature and activate the fan as often as needed, you will usually find this sensor close to the inlet water hose or thermostat housing (water outlet)
If you want to be sure that is the problem before you change the sensor and spend your money you can perform a little test take the plug off the sensor it has two wires get a piece of wire and jump them together with the ignition switch open , if fan starts after you close the circuit with the jumper wires you can be sure your sensor is gone. Good luck buddy. Blessings!