I suggest you start by reading some reference books that are regularly used on Certificate courses. By reading these books, you will be better prepared for the course, you will save time when you are actually studying and you will see if TEFL is really for you.
Some suggestions are: Jeremy Harmer’s Practice of English Language Teaching, Adrian Underhill’s Sound Foundations, Michael Swan’s Practical English Usage. If you contact the centre where you intend to study, they might send you the reading list. If not, go to a bookshop that specialises in EFL and ask them for suggestions.
You could also get in touch with people who have done or who are doing the CELTA. You could ask around your friends or consult websites such as this one.
Another way to prepare is to think about your own learning. Think about the teachers you liked, those who motivated you to learn and those who fostered a love for the subject. Think about their qualities or attitudes and consider whether you could do this yourself. Also think about the teachers who were not so successful; you can analyse why this was the case.
Finally, start preparing for your move to Hong Kong. Get as much information as you can about the place and prepare your move well in advance.
Good luck with your new career.