You're going to study in Thailand? Nearly everyone wants to come to China since Chinese and China is becoming really important these days. If you're going to study Thai, it's not really that important as virtually all educated people speak English....but anyway that's not the purpose of this post, I was just curious.

Go to www.thaiairways.com and book a flight online directly with them. Going to an agent won't save you any money and it's just a hassle and waste of time. Why bother with agents when airlines allow you to book directly with them? Alternatively, catch a bus down to Hanoi and fly from there as it's much cheaper (even with Thai or with low cost carriers such as Air Asia).

Zuji Hong Kong is also good as one previous poster recommended.