We are taking train from Shanghai to Hangzhou. Which train station do we take from and frequency/ cost?

You need to go to the Hongqiao Railway Station. The interval is around 30 minutes. If you take the D-trains, the fare is around 48.5RMB for the second class seat and around 59.5RMB for the first class seat with the duration around one hour and thiry mintues.
If you take a G-train, it costs around 77.5RMB for the second class seat and 123.5RMB for the first class seat with duration around one hour and ten minutes.