How do I study?

I'm afraid of not keeping up with the class because I really can't study. Even if I read a text a lot of times, try to concentrate and all that I can't seem to retain anything I literally don't understand a single thing

I live in Argentina so the last year before high school is the seventh, in that grade we learn equations, now I'm in second year in high school (I guess is the 9th grade) and I still don't understand them, I've tried asking my parents, the teachers to please repeat and all that but I'm really awkward so if they asked if I have understood I usually say yes even if I didn't

I'm afraid of repeating the year of something like that just because I don't know how to study, I don't even know how I pass the tests

Sorry for bad English :(