to travel from Hong kong Internation airport to Guangzhou... Pls advise transport detail

You can take the cross border coach from HKIA to Guangzhou directly. There are several different terminals in Guangzhou City, such as Garden Hotel, New Tianhe Hotel, Jiaokou, or Huangpu. Coaches are in service from 07:00 to 22:15. It takes about 4 hours, and the fare is 250HKD.

All the trains are sold out so I am scrambling for a bus. A friend took a bus once and was dropped off on the street and no one spoke English so I'd like to make sure that doesn't happen to me! Which bus company do you recommend? I'm staying at the Marriott Hotel Tianhe.

Well, as far as I know, long-distance buses of China Travel Tours Transportation are one of the most official bus company. You can take this from the airport, and get off at The Garden Hotel.
After arriving, take a taxi to your hotel. The cab fare is CNY 15 to 20.