If you’d like to take train to Guangzhou, you have to go to the Hung Hom Train Station first by airport bus A21 with ticket price around HKD33. And the earliest train to Guangzhou is departing Hong Kong around 07:30 and with arrival time at 9am. Or, you could take the bus directly to Guangzhou from the airport in case train tickets are hard to get. In Guangzhou, the city transportation methods include city-bus, subway, taxies and so on. About the details, you could refer to Guangzhou Transportation(https://www.travelchinaguide.com/cityguides/guangdong/guangzhou/getting-there.htm) and Hong Kong Transportation(https://www.travelchinaguide.com/cityguides/hongkong/transportation/) . In order to get the hotel rooms and train tickets, you would better arrange the booking in advance.