Can you post a link to the device you are actually using, as well as a schematic?
The MQ135-datasheet1.pdf file shows, on the first page, how the sensor needs to be connected to the Arduino. At a minimum, VRL and GND needed to be connected to the Arduino, AND a suitable value for RL needs to be selected.
You then take a reading with the sensor in 100PPM Ammonia. That gives you a value, Ro. Then, you take a reading in the unknown air. That gives you Rs.
You then use the chart on page 2, left side. The value of Rs/Ro is found on the vertical axis. Where the horizontal line intersects the colored line corresponding to the gas you are measuring gives you a point. The vertical line through that point tells you the concentration of the gas.
Note that the chart is not linear in either direction.