aurduino, fan pwm controlled by temperature sensor

how do i begin?

syrpimp89:
how do i begin?

Decide on which temperature sensor you want,

LM35 - Degree Centigrade 10…150 voltage 25degree = 0.250V
LM36 - Degree Centigrade -40 … 150 voltage 25degree = 0.750V

LM35, LM36 DataSheet

DS18B20 - Degree Centigrade -55 to 255, oneWire data protocol

ThermoCouple - K-type -200 to 1376 degree, SPI interface
MAX31855 thermocouple amplifier datasheet

Decide on your Fan,

If you want anything above a computer 120mm you will need a drive circuit.

Chuck.

Also check out my videos #14 and #15 which deal with motor and (eventually) cooler fan control in the Arduino.

The videos stemmed from my temperature controlled fans inside my computer cupboard (shown in the video) so it may get you started, at least, including a simple driver circuit.