"If I wish to go Don Muang Airport from Pattaya using bus, which bus station or bus stop in Bangkok should I get off from the bus?"
When you are in Bangkok you will have to use a taxi to complete your ride to Don Muang.
Maybe bus from Pattaya bus terminal to Morchit. Morchit is a little closer to Don Muang. Bus station is on North Pattaya Road.
See map for Pattaya. Locate North Pattaya Road. Bus station is on that road.
When using the meter taxi in Bangkok only pay the meter fare. Don't get in unless the taxi driver agrees to use the meter. I don't recall ever using a taxi from Morchit to Don Muang - if you only pay the meter (price is the same for 4 passengers but luggage room is limited) the ride is fairly priced. Have coins and small bills to pay the driver. Rough guess maybe 70-110 baht.
Bus from Pattaya does not run all night long! Last bus may still be 9 pm.