Hello Mr. Tan:
I am afraid that there is no any bus from Shenzhen airport to Guangzhou directly.
So please first, take the line 330 at the airport to Xinhua Hotel ,it costs you 20RMB and it takes about 35minutes to get to the Xinhua Hotel.the bus line 330 comes once in every 20 minutes.
Then,There is the subway station named Kexueguan right in front of the Xinhua Hotel , you can take the subway to the Shenzhen East Railway Station at Luohu. It takes about 10minutes to get to Luohu and it costs you 3RMB.
And then, you can take the train to Guangzhou, the ticket for Guangzhou is 75RMB and it is more than 150km from Shenzhen .It takes about 72minutes to get to Guangzhou by train.
For further information, you can call the airport hot line 0086- 0755-23456789

Good luck!