Just read it over and over again. If there is no foundation before, choose the introductory textbook. It's not too difficult. In fact, Gao Hongye's is already very good, which is very good as an introduction. If you encounter problems in self-study, Baidu will be fine. Really feel the pressure of self-study, just sign up for a class, not the society, and waste money. Classes will be held in ordinary schools. You'd better go to the school you want to take, or consult the graduate students in that school.