when is the first train or bus from guangzhou city to airport as my flight is at 7:30am

As I am not clear about your location, it is hard to tell which lines you will use. Anyway, the first train of line 3 to the airport leaves at 06:00, but it is not early enough to catch ur flight, because you need at least 2-3 hours to check in. Therefore, I would suggest you take a taxi to the airport very early.