visa

Hello, we are USA CITIZENS with a Thailand visa that would be entering the country through flight from Bangkok but we have heard that in order to enter China for this 4 day tour we would need to acquire a entry visa before entering China or boarding the flight unlike other countries where you just land and get your visa stamp when you land if a US citizen.? Can you tell me if this is true? And do you have recommendations on the quickest and easiest way of doing this if so?