How do I "confirmed the seats" to avoid a transit visa

Hey we are a family comming from thailand the 5th of january to Guangzhou and 1,5 h later the 6.th of january at 00:20 we are going further again with China Southern Airlines. I think we do not need one but the chinese embassy told me to get "seats confirmed" How can I contact the Airlines??I cann´t find a number. Now I only have the number of the flight and the time and my reservation is done- Is that enough?

regards a