This circuit is a simple circuit where the speed of the fan is controlled by temperature. You require a pot of 1 kilo ohm and a thermistor. Use a NTC type thermistor i.e. the resistance will decrease when temperature increases.
Hence the overall impedance of the circuit will increase. Therefore the current flowing through Fan motor will decrease. Due to this speed of the fan will decrease. If you want to vary the speed of the motor individually change the R1. SW1 is used to turn on the fan. SW2 is used when you don't want temperature controlled mode for the fan.