Hi,could you tell me what bus station in Shanghai is the best to catch a bus to hefei?

Both General Bus Station and South Bus Station have buses that can take you to Hefei.
The schedule of General Bus Station is at 08:40. The ticket fare is CNY 110 per person.
The long-distance bus from South Bus Station departs at 10:01. It costs CNY 160 per person.

BTW, there are also both ordinary and high speed trains from the Main railway station and Hongqiao railway station to Hefei. And the departures are much more frequent than the buses. You can have a look at the detailed schedule by searching this website.

Thanks you very much sir for help,sir how long time I need this journey,what time bus departure and arrive

If by taking long-distance bus, the duration is about 6 hours.
There is only one schedule respectively for General Bus Station and South Bus Station: 08:40 for General one and 10:01 for South one.

Trains depart from 06:09 to 23:36.
Bullet train spends around 3.5 hours, while ordinary train needs nearly 6 hours.

Thank sir can I buy a train tickets at the same time,tickets is available or seme time sell out???

Yes, you can buy the bus ticket and train ticket at the same time. But it will be a waste of money, as you can only travel by one of them.
I don't know your departure date. So I cannot find out whether the tickets had already sold out.
But you are strongly advised to book tickets in advance.

Thank you sir ,sir my departure date -6sep..afternoon so can I buy train ticket that time,ticket is available.what is the fare?

There are still lots of tickets available till now. But the leftovers are changing all the time. You can keep an eye on this website for the availability of the tickets. I personally think you should better book the tickets in advance to ensure a smooth trip.

About the ticket fare:
High speed G train: CNY 300.5 for a first class seat, CNY 203.5 for a second class seat;
High speed D train: CNY 217.5 or 187 for a first class seat, CNY 156.5 or 151.5 for a second class seat;
Regular K trains: CNY 237.5 for a soft sleeper, CNY 152.5 for a hard sleeper, and CNY79.5 for a hard seat.