(Cross-posted in MESE 8173.)
I want to start to do mathematical Olympiad type questions but have absolutely no knowledge on how to solve these apart from my school curriculum. I'm $16$ but know maths up to the $18$ year old level. I think I will start learning the theory of the topics (Elementary Number Theory, Combinatorics, Euclidean Plane Geometry) then going on to trying the questions, but I need help in knowing what books to use to learn the theory. I have seen several list (such as http://www.artofproblemsolving.com/wiki/index.php/Olympiad_books) but does anyone know which ones are the best for my level of knowledge.
P.S. I live in UK if that matters.