How to prepare for International Biology Olympiad?

If you have limited knowledge, firstly I recommend you to read books that help you prepare for you national biology exam (sounds like you already have). They cover general biology topics and do not go into details. Also Campbell biology is the best book to start learning upper level biology so read it carefully.

From my own experience I can tell you that the first time of reading any book is quite daunting as you do not understand certain things. But the more times you read, the better understanding develops. Thus, not only read a book once and say 'I'm done with it' but after one or two weeks read it again, then once again and again. In this way the information will solidify in your head and you will need less effort to retain info.

I found here a super huge list of textbooks: http://biolympiads.com/book-list. I think if you read them, you should do fine. And also do past papers - they are essential!