There are three options, one is take direct ferry from Nansha Ferry Terminal but there are only two ferry leave at 11:40 and 15:50 in a day and you need to take bus or taxi to there.

the second one is to take direct train at Guangzhou East Railway Station to Hung Hom, Hong Kong. You said you are near the raiway station, if it is the East Railway Station, you can buy the direct train tickets there, if you are near Guangzhou Railway Station, you can take subway or bus to East Railway Station and take direct train.

The third one is to take train to Shenzhen and pass the customs and take MTR to downtown Hong Kong.

In downtown Guangzhou, there are many buses departing to Hong Kong Airport directly. Walk down from Guangzhou Railway Station, you could find the Dongfang Hotel in 20 minutes. In front of this hotel, you could catch a coach to Hong Kong Airport directly respectively at 06:31, 08:30, 10:00 and 16:01.

Well, it is suggested that you visit the official website of Hong Kong Airport for the bus schedule in detail.