nanjing to beijing

Do you mean Hong Kong?
there is no direct rail service from Nanjing to Hong Kong. but there are many high-speed trains from Nanjing to Beijing daily. so you don't need to book in advance if you don't care the travel time and classes of seat. usually second class is sold out fast. but i still suggest that you buy the ticket as early as possible.
your passport is required when you buy the ticket at railway station.
railway station is always crowded with people, and you may need to wait in long lines to get a ticket, so you'd better to the station early.