How should I prepare for international-level Olympiads?
If they are like the International Mathematical Olympiad, then they lean toward problem solving ability over knowledge, so learn a few basic principles then do a lot of example questions.
I can only speak of the International Mathematical Olympiad as that is the only one for which I prepared, and that was nearly 40 years ago, so things may have changed a little :-)
The aim of the questions seemed to be not to test that you knew many facts about the subject, but a few basic principles (e.g. the pigeonhole principle, modular arithmetic) and a lot of lateral thinking.
It is possible to get hold of past questions. I would advise just spending some time practicing these in order to get a flavour of what to expect.
I can only talk about Physics and Chemistry, since those are the two Olympiads I'm fairly familiar with, but I echo what George C said - International Olympiad problems tend to focus more on understanding and problem-solving skills than on knowledge.
The interesting thing about these two Olympiads is that you'll end up using a handful of concepts in a wide variety of ways.
So, in essence, you'll spend most of your preparation time juggling with a few key concepts instead of learning new ones.
At this level, you must be able to derive any formula on the spot. This helps a lot when it comes to assimilating a concept. Try to avoid learning things by hart - that is not a useful approach to have at this level - or at any level, for that matter.
The problems set includes most of the formulas and all the constants you'll need anyway, so focus on playing around with those formulas - think how does changing this affect that?.
Now, I think that in order to reach international level, you must focus on one, maybe two Olympiads. You are really, really young, so use this first attempt to figure out which of these three subjects you enjoy the most.
Because it will eventually come down to which subject you love - you can't prepare for this type of examination without having a clear interest in the subject.
Also, keep in mind that both the Physics and the Chemistry Olympiads have practical sections as well. This means that you're going to have to spend a lot of time in the lab learning how to set up various experiments.
The great part about that is that you'll get to see how the theoretical stuff came about, and you'll love that subject even more.
In this regard, support from your school is a must, so make sure that your teachers are
It's always a good idea to check out the problems given in past years. If you can, try to speak with students who already have some experience at that level. You'll hear a lot of "forget about that , focus on this"
As a final note, it would be great if you could commit to this for a couple of years.
Saying that this is your "first and last attempt" at reaching this level puts unnecessary pressure on you to succeed
Of course, I don't know what your situation is, but I definitely suggest giving this a couple of more tries.