How do I prepare for the International Earth Science Olympiad?
Here are a few, completely random advices I wished someone would have given me before I started to study geology. I'm aware of that this is over simplified, but I believe that it's good to start easy, gain interest and later, with experience, understand that you have been wrong. 

A good book to start with: 
Earth: Portrait of a Planet by Stephen Marshak. It is used in many universities for undergraduate students. It has a lot of good illustrations and goes breathy through most aspects of earth science. I'd check that and memorize the diagrams and pictures. 

http://www.usgs.gov/ also have plenty of easy accessible material. 

Make sure that you understand the concept of tectonics well and use that as a starting point for almost any geological discussion. 

Don't forget the most basic physical properties. What works in a laboratory also works in larger scale. (E.g. Density will make light material float and heavy material sink. That's why rivers erodes the paths to the oceans and volcanoes erupt volatile rich lava. Oceanic crust is heavy and continental crust is light.)

Take one hour of your life to learn the geological periods (everyone should!). Use the periods as imaginary folders for fossils, orogenies, mass extensions and whatever you later come across. (E.g. 'Permian' will be something distinctive, just like 'Elephant', 'Qawwali' or 'Andhra'. You'll recognize it when you see/hear/feel it, even if it's difficult to describe in words!). 

When you read articles, always start with the latest, and work your way back. In this way you'll start with the modern concept and not get biased by earlier models. 

Keep an interest in the present. You should know some biology to understand paleontology, some oceanography to understand deposition and hydrology to understand erosion etc. When you add time, we call it geology. 

And, if you start to study and research in geology or anything else, learn LaTeX and learn some basic programming, before you need it. 

Good luck with the Olympiad!