Which train I have to take from Hong Kong airport to Guangzhou Baiyun area?

You need to take a ferry from the airport to Nansha Port first and then take the metro line 4 from Nansha Port Station to Huangcun Station. Get out of the exit B and walk about 150m to Olympic Sports Center General Station to take bus line 60. Get off at Guangyuan Bus Station; you just arrive in Baiyun area.