The system would work somewhat like this:
Temperature sensor (thermistor) would give you an analog value which you will compare with a pre-defined voltage level (set using a potentiometer) using a comparator and if the output of comparator goes high it will further switch on the relay, using a transistor, which will operate the fan.
I would suggest you to use following components:
1. LM35 temperature sensor
2. LM358 opamp as a comparator
3. BC547 NPN transistor for swtiching
4. 5V relay
Now talking about the resistors and all:
First of all connect a 10K potentiometer(random value, this value doesn't make any difference as such) to the inverting input side of comparator and LM35's output to non-inverting side. Then conect the output pin of this comaparator to the base pin of BC 547 trasistor via a 1K resistor. Pull-up the collector side using a 10K resistor and ground the emmiter pin. Now connect the collector of BC 547 with a relay pin and the other pin of relay with VCC. Also try connecting a diode in reverse biased mode to these relay pins, this would help you to overcome induction problems of the relay.
This is how you could proceede to build the above mentioned stuff.