npn transistor + thermistor

If you and your friend do not even have the basic idea of how transistors can be used to switch electronics on/off, the whole project might be taken up for nothing.

What you basically need is a temperature sensor, and this you can use as ckh93 suggested - a thermistor.

Then you need a switch to turn the fan on/off right?
For that purpose you can choose a transistor because they are cheap and robust, and NPN is a common configuration so there are lots of schematics on the internet.

Basically, just take a look, scout around, try and learn stuff for electronics.
Spoon feeding does not bode well for people who want to learn electronics rather than learning the basics first then expanding your knowledge by building your own stuff!

Try.

Cheers!