Lots of possible ways to do it. Forr example, if on/off type of control is on, you can use a comparator, hook it to your sensor and compare the ssensor output to a present value to turn on or off the motor.Please help
If you want pwm type controls (the fan runs faster at higher temperature), use a pmw controller, a 555 for example could do the job.
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All depends on what you want.