The Hydrogen sensor is an analog device, so you will need to use an Analog input on your Arduino, run a sketch to track the signal voltage from the sensor (calibration, alarms, etc. Can be done here too) and then when the signal is higher than your preset, lower a digital output pin connected to a transistor/FET to activate the relay coil that will turn on the 12 V fans.
Here is a wiring diagram for the device:
Arduino stuff on this site in this location: playground.arduino.cc/Main/MQGasSensors