from gpiozero import LED
#we are using the LED sub-module just as a generic output
fan = LED(18) #for the positive, put the negative in one of the grounds
def cputemp():
f = open("/sys/class/thermal/thermal_zone0/temp")
CPUTemp = f.read()
f.close()
StringToOutput= str(int(CPUTemp)/1000)

while True:
cputemp()
if StringToOutput >= 45:
fan.on()
elif StringToOutput < 45:
fan.off

This is some fairly simple code that gets the temperature from /sys/class/thermal/thermal_zone0/temp in thousandths Celsius divides by 1000 for Celsius and checks if it is more than 45. if it is it turns the "fan" on and if not , it stays off. But you'll almost certainly never need a fan as long as you're not doing anything stupid.

This code was adapted from a SE question but i am not sure which.