In the most big cities there are two or more train stations, such as Beijing, there are 5 train stations named Beijing, Beijing East, Beijing South, Beijing West and Beijing North. You should find out which train station is the right one to board your train in advance. Else you are on the missing way at the beginning.
You should arrive train station at least 40 minutes in advance to get yourself enough time to board your train. To enter train station, first you must go through security control, you have to put your bags through an X-Ray machine. It may take a few minutes to wait in line for there're usually many passagers.
After passing the security control, you need to go to the waiting room where you will board your train. Usually there's a big scrolling electronic message board shows waiting room info for each train. You also can ask train station staff where your waiting room is. In most train stations, there's a separate waiting room only for passagers who have soft seat / sleeper tickets, and preferential boarding is available.
About half an hour in advance, you will be adviced by broadcast or electronic message board to check in. The staff will check your ticket and then you can go to the platform.
The carriage number and seat number are shown on your train ticket. You can easily find your carriage. After boarding the train, if you have a sleeper ticket, you are asked to change your ticket to a plastic card, around a hour before you arrive at your destination, it will be changed back. In your whole journey, please keep an eye to your belongings, and take care of your train ticket, you need to show your ticket one last time when you exit the station.