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

You must refer to the Pudong airport, right?
If yes, you can take the airport shuttle bus line 1 to Hongqiao Train Station directly. The bus fare is CNY30, and the travel time is about 60 – 70 mins.
If taking a taxi, the fare is about CNY230 – 240, and the duration is also about 60 – 70 mins.