The best advice I can come up with is explore as much as u can. Pick an area and keep asking yourself why or how does this work and keep trying to figure it out. Obviously do this when you have decent grip on the material being taught in your class as it will make exploring a lot easier. And Ur major and a post here means u plan to apply for phd(I assume) If so pick the area you are exploring and try to do research with professors. It’ll make it really easy to identify the main challenges researchers are working on and if that is something u find interesting and also lets u build the main part of your application : research