Relax, its just a Honda City. Just maintian it like any other car, making sure you change the oil on time, check the water/coolant, get the car regularly serviced, prefarably from an authorized dealer and use genuine parts.
If its a maunal and driven with care, you should get about 14-15 KM/litre in the city and more on the long drive. I dont know about the CNG average though.