Bus time table from Beijing to Datong (to-from)

Hi,
we plan to take bus for the trip (4-5 hours journey) on 6th Sept,2014. Can anyone please advise how and from where to purchase the to-from ticket in BJ , where is bus station located in the city, upto what time the bus station is open, at what time is the last bus to Datong and for the return journey available. Also, please advise whether we can book bus tickets from Shanghai or we have to book from Beijing bus terminal only. There is not much information available on internet.
Thank you very much for your help in advance.