You have to queue up at the ticket office, which may be in Chinese characters "售票厅" or "售票处", with your passport (or other identity document you used for booking) carried. The "售票厅" or "售票处" can be easily caught outside of the office building, and inside the office building there will be several ticket windows. You can make a ticket collection at most of the ticket windows except for the alternation windows (改签窗口) and cancellation windows (退票窗口). There may also be a separate ticket window to collect train tickets booked online, but the waiting line may not be the shortest.
Only selected staffs in metropolises like Beijing and Shanghai may speak English, so remember to queue in front of the right window or else your tickets may be canceled for the language barrier. You can also simply show the below sentences to the staff:
● English: Hello, I have booked my High Speed Train tickets online, is this correct waiting line for collection?
Chinese: 你好,我在网上订票了,是在这里取票吗?
● English: Hello, I want to collect my ticket. My passport please. Thank you!
Chinese: 你好,这是我的护照,我想取票。谢谢!