What grade are you on currently?
I had moved schools, so I changed exam boards. As a result, the grades I often got dropped a bit (got a 7 in one test).
What I would say is to first know your exam board. Then, look for the syllabus somewhere -if your school uses a certain website,eg mathswatch that should have it. If not then use seneca and just go scroll through each topic (not in depth, just the title). Make a list of what you don't know. You could confirm your knowledge of the other subjects you're shaky on by doing some questions on them, but definitely don't do questions on topics you are confident on.
From there, just learn the content. 'Understand' is the key term in Maths- unlike most other subjects, you don't need to memorize nearly as much : perhaps a few equations, but that's all. Understand HOW to do each topic. For example, how to do functions. If you don't understand them, there is no shot at scoring marks.
Once you understand everything in the syllabus (which isn't as hard as it sounds), do practice papers. It's commonly said just practice practice practice because that's the best (and probably only) way to ensure success.
Good luck my friend