I need to go Hong Kong from Guangzhou Airport.

The shuttle bus should be Line 6A which could drive you to the Tianhe Terminal, located in the walking-distance to Guangzhou East Railway Station. However, it is always difficult to purchase a train ticket to Hong Kong on the spot at weekend and holidays.

Alternatively, at Guangzhou East Railway Station, you could purchase a bullet train ticket to Shenzhen Railway Station easily. It will take you about 70 minutes on the way. Upon arriving in Shenzhen Railway Station, please follow the directions to the Luohu Border, go through which you could enter into Hong Kong.