As a 13 yrs high school math teacher in the past, I can tell you that taking notes during a class, then studying them will really help, as well as studying the book or papers you have. Study in a quiet place; no TV, no music, no phones, no electronic equipment, unless you need something to look up stuff. Noise is a distraction, no matter what it is. Good luck.
When you have read a paragraph or section, see if you can put it into your own words as if you were explaining it to another person. Perhaps even explain to an parent or other family member. If you can explain what you just read, then you yourself have understood it.
This is going to sound sssooo stupid but, if you read things in different Colours such as putting coloured cellaphane (did I spell that right?) is supposed to help. Or writing it out a lot in different coloured pen.