Where can I buy train ticket in the internet?
I understand that there isn't such a service in general. Train tickets can only be reserved at the station itself between 3 and 10 days in advance, and cannot in general be purchased outside of the departure station. Your best bet is to buy a ticket from the departure station as soon as you get to that city.
And I wouldn't worry too much about getting the ticket far in advance. You can almost always upgrade your ticket once on the train as there are often many empty spots available (so just buy whatever ticket is available even if you can't find the right class). The upgrade cost is the difference in the prices of the tickets, plus some tiny surcharge. In any case, you can often get a ticket for a train departing the same or next day.
No matter where you go... avoid the Chinese New Year...
The Golden Weeks are also very busy ....
Golden week is the one starting tomorrow (October 1), right?
Why is it called Golden week?
China now has three "Golden Week" holidays in the year -- Spring Festival, International Labor Day, and National Day. This new holiday scheme was set up by the Chinese government in 1999 so as to boost domestic demand, stimulate consumption, and restructure the economy. As the prolonged holidays turned out to be "golden" for the economy, they became known as "Golden Week".
In other words, where the masses go and spend their money in one blow and afterwards work their ass off for the entire year again.
I should have probably also pointed out that for larger departure cities, you can find pretty much any travel agent based in that city who would be happy to take a hefty surcharge and payment in advance in exchange for going down to the train station a few days in advance and buying a ticket for you.