Since this question has been posted in Educators stack exchange, I would try to structure my response in a way that addresses different learning approaches.
Approach - 1 : Learn techniques first, apply learnt techniques
This is the most traditional approach. Let us take the example of number theory. The chapters from a typical number theory book look like this - Introduction, Prime numbers, Congruences, ...
You start with the prime numbers chapter and go through every method, technique and understand the applications,then start solving exercise problems belonging to that particular chapter. Then move on to the next chapter.
While this approach is good, I am a bit concerned about the way it affects the development of your problem solving abilities. Some of these chapters may take days to complete and the exercises at the back may focus exclusively on a few select topics.
Approach - 2: Learn enough theory; work on problems; go back and learn techniques
Learn enough theory to understand problems. For instance, you can't solve congruence equations without having knowledge about congruences. Grappling with problems at an early stage is very advantageous. After you spend sufficient time on a problem and are unable to get any ideas, go for the solution. This is a good way to track your progress. Initially the progress may be very slow and frustrating, every problem that you try may elude a solution. But gradually you would start doing better.
My suggestion would be to look at a book of problems very early on to see where you stand. If you are able to follow the solutions without too much difficulty, this would be a good indication of your aptitude for olympiad math. This is a short book by Titu Andreescu that has a good collection of problems and enough theory to back them up.
http://www.amazon.co.uk/Mathematical-Olympiad-Treasures-Titu-Andreescu/dp/081768252X
A few other resources that might help:
Link
http://files.vipulnaik.com/olympiads/preparingforolympiads.pdf
Caveat: Though hard work is very important, a certain level of problem solving ability and intuition are indispensable when it comes to solving Olympiad mathematics. You develop them as you work. You are up against some of the most competent problem solvers. If you are doing good enough at school, I think you must certainly go for Olympiad mathematics.
