It is better for you to buy the tickets in advance in case there are no tickets left when you reach the station. From the airport you can take the airport shuttle line 3 to the train station, usually it costs about 1hr and 20 mins. However, time is longer if encountering the traffic jam. For saving time, you can also choose to take the airport express line to Dongzhimen first and then take the subway line 2 to the railway station.