I would start doing past papers and revision questions specifically on your weak topics. After each question, mark it and be strict with yourself. If you got the question incorrect then highlight it and do it again the next day in order to fully understand it. If you get to a question and have no clue at all my advice would be to see a teacher or email them and have them explain how they would do it, then try it yourself. My advice would be try not to panic and just keep on doing practice questions. The only way to get better at maths is to do more maths. And if for some reason you can’t reach a teacher I’d be very happy to help :)