Shanghai to Hong Kong 14 July 2016

5 questions.
1) i need to get to HK because my flight home departs from HK international airport. What documents are required to book the ticket and will the HK immigration require anything apart from passport (i own only a chinese visa)?
2) how far is the train station in HK to the international airport? Are there metro/coaches or any other very convenient and quick means of connection? If yes, how much time could that take? And how much could it cost?
3) i need the ticket for July 2016. How early can i book the ticket (the earliest the better).
4) are their any baggage limitations?
5)i'm already in China (Nanjing). Where can i easily buy this ticket (store or online)?
Thank you!