there is a nice arduino-related tutorial for MQ 135 in http://playground.arduino.cc/Main/MQGasSensors
Assuming it is this sensor mentioned there, it provides a single voltage output only for it whatever the sensor is measuring.
Unfortunately, this voltage is 0 to 5V (at least it is possible to be in this range), so you need
- an ADC-Converter like MCP3002 or MCP3202, which is inexpensive, running from 3.3V
- a voltage divider on the input, which brings down 5V to 3.3V
The load resistor can form a voltage divider when you use two resistors in series, for RL = 20kOhm, the 'ground resistor' is 13kOhm and 6.8 kOhm to the sensors output. Be sure the voltage divider is build 'rock solid', specially the ground connection in order to prevent 5V to flow into precious ADC.
Now the question has changed to 'how to connect and run a MCP3002, MCP3202'.
On my web page, there is a sketch on how to connect the MCP3202 (text in german) http://heppg.de/ikg/wordpress/?p=134
There is quite a lot of sample code for these devices.