could you tell me which bus i must board from Shanghai international airport?

Do you refer to the long-distance bus station located at No. 1015 North Zhongshan Road?

If yes, You can take the airport shuttle bus line 5 to the SH railway Station (about 70 mins), and then take a taxi to the bus station at a cost of CNY15.

The bus ride to Zhenjiang is about 3 hours, and the fare is CNY76 around.