Shanghai Railway Station has two trains to Guangzhou and Shanghai South Railway Station has four trains to Guangzhou every day. Both of the train ticket departing from these two stations could be purchased at any of them.
There are multi-buses leading to the two stations, but I suggest that you take the subway which is faster.
To Shanghai Railway Station: Line 1, 3, 4
To Shanghai South Railway Station: Line1, 3

As for the bus information, please refer to: https://www.travelchinaguide.com/cityguides/shanghai/transportation/train.htm