When youa rrive at Guangzhou Railway Station, please hire a cab driving to the China Hotel (中国大酒店)or Dongfang Hotel (东方宾馆), where you could take a coach to reach Hong Kong Airport directly.
As for the bus schedule and ticket booking, please visit the official website of Hong Kong Airport: http://www.hongkongairport.com/CBT/Controller?locale=eng
Ps: these two hotels are located no more than 3km away from the train station, so the cab fare should be within CNY10.