Nearly all tickets sold in China are single journey tickets. These will always come with an included seat reservation unless you hold a Standing Only ticket — which is only possible for HSR Second Class, or Hard Seat on Regular Rail.
To buy a single journey ticket, you need the following ready:
▶ Date of Travel
You must know on which date you will be leaving.
▶ Train of Travel
The train number will determine what time you will travel — and vice-versa.
You need your destination station — the exact full name — and if you are buying a ticket from another station, your boarding station as well.
▶ Class of Travel
Usually, they default to the standard class of travel — Second Class on HSR, or Hard Seat on Regular Rail, but you can change this here (and you should).
▶ Carriage and Seat (Optional)
You can specify the specific carriage and seat you wish to take whilst on the go. You must know this ahead of time, though, since the member of staff behind the ticket counter might not know this very well!
▶ Your identity document (All trips)
Except for trips wholly on the Beijing Suburban Railway Line S2, or the Jinshan Railway,
Always have alternative travel plans ready. Chinese trains carry literally thousands per train, so you might need to book onto the next available train, especially if you were thinking of travelling on a popular service.