I think you drew that wrong.
There is a thermistor in series with a 10k pot between +12 and ground. When the thermistor resistance is low enough that the voltage on said divisor exceeds Vgs the mosfet turns on.
12 volts goes into the fan, the black wire on the fan goes to the drain and the source goes to ground.
http://www.irf.com/product-info/datasheets/data/irfz24vpbf.pdf