Changsha south is the station for the bullet trains. Not particularly difficult to figure out except that there are only 2 ticket counters in the ticket office. See where to find other ones.
Visit our channel for more videos about train travel and train stations in China: http://www.dailymotion.com/china-diy-travel-video
To consult online timetables and book tickets, visit: https://www.china-diy-travel.com/en