Let's start at the start. You are right, many people who apply for the CX cadetship are rejected. I assume you are a HK citizen, otherwise you aren't eligible for the CX cadet program.
CX spend a lot of money training cadets so they can be picky. Im sure there is plenty of info out there on what to study and how to apply. I know a few people in CX and none have a degree.
I assume you are still in highschool so the best thing in my opinion to do is order a private pilot license theory book to get familiar with avaition terms and basic flight theory. As I said before, there are many "CX study guides" online you can refer to for guidance. The PPL book won't cost much and you find out fast if your not really that into it.
Going to be a while before Cathay recruits again so plenty of time to get ready.