I Am From Croatia And I Would Like To Go On Universities Like Oxford, Yale Or Harvard What I Supposed To Do?

This year I am going to high school and I am thinking between MIOC (on English: Math and Informatic Education Center ) or language high school (there is on first place to learn two or three languages). In Croatia you can’t choose your subjects you need to go on all of them and material here is so hard and large. I know that for all of universities you need A. Here to go on universtity you need to have big percentage in state graduation. So here is thing in Croatia you have all subjects in school you can’t choose subjects (here I have 16 subjects with foreign language ), you don’t need to have A in high school to go on good universtity. I know I want to go on law school or medical school. If I go to MIOC I will have excellent knowledge for medical school (from MIOC you go to best universities in Croatia). So I don’t know what you need to go on that universitie in America or England. What subjects in America you need to take or how big is your material. Here from history for example we learn history of world. From stone age to Napoleon and Staljin…everything. I will have knowledge but I don’t know what people who go on Yale, Harvard or Oxford learn in high school. What subjects they take. It’s my dream to go on universities like Harvard, Yale and Oxford.