I would recommend Trip.com. They add on a small booking fee (about £3 per ticket), but for the ease of being able to use international credit cards and a decent user interface, it's the reliable choice for me. My seat preferences (direction and side of the train) have always been successfully granted too.
Booking in person at the station is also possible, but there's the risk that your preferred train will be full, and if you don't speak any 中文 then you might struggle at the ticket booth. The Trip.com mobile app produces a screen with your route and confirmation number, that you just need to show at the counter, no speaking needed. I'd recommend taking a screenshot of that in advance in case you don't have an internet connection at the station.
If this is your first time travelling by train in China, have a read of Seat 61 (
https://www.seat61.com/China.htm) too. The process is a bit odd the first time (security checks, showing your passport 3 times, queuing up at the boarding gate and then again on the platform), but it's simple enough when you know what's going on.