The sensor MQ -135 works with many gases, I get the concentration of CO2
No you don't.
You get whatever gas the MQ-135 is reacting to. It could be CO2 but it could also be any of the other gasses or the sum of several gasses. If you calibrate your sensor with CO2 then you calculated a value as if it were all CO2. Don't expect these sensor to be very accurate.
Out of the box these sensor are "poluted", with water, other gasses. It takes many ours of heating before the sensor is clean (burning-in). During this time R0 will get higher until it is relatively stable. If you use the sensor again within a couple of days it will take less time to clean.
For calibration you need at least two gas-mixtures with known CO2 concentration, preferably the lowest and highest concentration you want to measure, one extra in the middle would even be better. For the lowest calibration you can use outdoor air (400ppm). Getting something cheap for the higher concentration is more difficult, maybe you can use 10 times diluted exhaled breath (4000ppm). Breath is about 4% CO2 (40000ppm).
Regards,
Alex