Either go to the train station or use www.12306.cn (the official site) as soon as you use any other online ticket agents you are at risk of getting scammed. Note, as a foreigner you need to actually go pick up your ticket whereas Chinese nationals can just use their ID card (at least for the high speed trains) 

You should realize (or know) that train tickets go on sale from 14 days to 10 days prior to departure, depending on which city you are located at.

You should realize (or know) that train tickets go on sale from 14 days to 10 days prior to departure, depending on which city you are located at.

Once you are aware of this fact, then only buy train tickets from either directly at a Train Station (almost all the time long lines, long waits), or at a Travel Agency that sells them. You will need your passport to do that.

I always buy them after 6 pm on the day prior to the tickets going on sale, and I always get what I want. And it just cost you 5 yuan extra

Go to the ticket window it's the best way. You could do it another way but that would probably be your best way of handling this..

aha! As a Chinese, it's quite difficult even for me   Hard sleeper tickets almost become legendary, especially during Chinese lunar new year. My advice is using 12306.com and some ticket-grabbing software. Good luck!

Below are some of the options I can suggest:

1. Use the official railway website 12306.cn. You have to create an account before you can do that. After you've selected your ticket, you need to pay online. And then on the day of departure, you go to the train station and pick your ticket up. Remember to bring your passport along.

2. I'm not sure which city you're in, but in the bigger cities you can buy a ticket at any one of the tiny train ticket offices near you. You pay an additional 5 yuan for transaction fees. Also, remember to bring your passport in order to buy your ticket.

3. Go directly to the train station and buy a ticket there.

4. Book through a travel agency.

Happy traveling!

P.s:

If you're not traveling too far, maybe you can try taking a bus instead? If you're not freakishly tall, the sleeper buses are prettty comfy.

i use a agent where the shop sells train tickets for 5 quai commission its pretty reliable and saves you waiting at the station in ques. Juts go 10 days before your planned travel and early in the morning in case they sell out. But if they do maybe pay a little extra and go 1st class as generally have more available.

Use a travel agency on line. Thats what I did the first time. The second time I couldn't get tickets from one in person, so I flew. I bought air tickets at the last moment easily but couldn't pay for them with a foreign credit card because there was less than 24 hours left.