You get together with the teacher after class and let them know you are having trouble. It is their job to help you understand. You might ask your parents to schedule a meeting with the teacher to work out a plan for your studies. You might have a different learning style, or you might have a learning disability, not understanding things does not mean you are stupid, but you have to ask for help (as you did here). Good luck.
Passing exams comes from the person who is supposed to be sitting for the paper. If you have the mentality that you cannot pass the exams, then you are likely to fail. If you feel you do not understand anything, the best thing you can do is to find a tutor and make your own timetable. This will help you grab some concepts which will definitely add you some marks. You can talk to your teacher for assistance.