practice on all of them , nothing force you to choose only the best and practice on it

Obviously doing USACO problems seems to be one of the best choices you have for olympiad training. Problems there are more likely to be olympiad-oriented and are more focused on IOI syllabus. One of the minuses, however, is that USACO problems aren't archived well. In my opinion, their training system is a bit outdated and presents problems in a rather random order, which could lead to disappointments and confusions . You could also check their old contest problems as they are presented along with detailed analysis, coded solutions and test data.

I would also recommend checking other high school contests. For example, the Croatian COCI, or this Polish website. Other alternatives include Indian olympiad online judge, in addition to a set of nice articles on the required theory.

Once you feel you have a solid background, you can search for CEOI and BOI problems and attempt them. Test data is always available. Of course, you should get used to the new IOI style by attempting problems that have appeared on IOI 2010,2011,2012 and 2013. Some of CodeChef problems have a very similar scoring system to the new IOI style, where tasks are divided into sub-tasks of varying difficulty.

Undoubtedly, you should participate regularly on CodeForces, as it is the leading Competitive Programming platform right now.