i dont know how to convert the output to a useful one
If by "useful" you want a CO value in parts per million you have several hoops to jump through.
You will need to measure temperature and relative humidity. If the temperature is not 20°C or the relative humidity is not 33% you will have to adjust your readings according to Figure 4.
You also have to calibrate the sensor with a known concentration of CO. The R0 value (resistance at 100 ppm CO, 33% RH and 20°C) is the basis for measuring other CO levels and can be anywhere from 2KΩ to 20KΩ. You have to measure it and to do that you need a chamber with a known concentration of CO, preferably 100 ppm.
If you just want a VERY ROUGH value, look at my first response: "if the resistance drops to 1/28th the resistance in clean air you probably have about 100ppm and 1/280th would put it at about 4000ppm"