my questions are:
1. in order to bring fuel needle up...do we need to switch off car at petrol??
2. or we need to start car at petrol??
Dude, you are not suppose to do that reason being, you got an EFI CNG kit so it starts on petrol whenever you start it and will be converted to CNG automatically.
The fuel needle is very proud actually, it goes up whenever its in a good mode. 
My basic point was that, whenever you top up your fuel tank to full and then start driving on CNG, the needle will go down as you are running that car on petrol but after like 1 week time or so, it will go to its actual position when you switch your CITY on. So now its up to the needle when it goes up to the actual position...
Th logical reason behind this needle issue is that CITY 2010 ODO meter is incorporated with Electronic mileage calculator which can only be used on Petrol but not CNG. So that is the main reason of Fuel gauge issue on CNG.