If you know a little Chinese, and have an online banking account (if you have bought stuff on TaoBao before), these screenshots will make it very easy to buy your own train tickets:
For the first time, you need to create a free account on the website, the registration link is shown on the second image... The registration page screenshots are not included, but registration is as easy as filling a form (use a dictionary for that part) and clicking on the activation link sent to your mailbox.
After these steps, your ticket will be reserved, and you will have 40 minutes (or so) to pay for it. If you don't pay in time, the ticket will become available to others. Also this is an E-ticket. meaning you can go to the train station an hour or two before your trip, and hand in your passport and your E-ticket number to the Ticket office to receive your boarding card ;-) The trip information will be sent to your email and phone number via SMS.
Note that most of the tickets, if not all, (the cheaper ones anyway) are non-refundable and non-transferable to others.