I haven't done Chemistry but I got an A in both biology and maths. For biology, I would recommend making flashcards ASAP after each lesson (I used anki cards as I found a pokemon addon to gamify my experience), and then do exam questions. Exam questions you found difficult/have memorisable answers (like processes and definitions from the mark scheme) can be written down/typed into a doc so you remember their answer and put this into the flashcards as well. For maths, do lots of exam questions. Also, make cheatsheets and mindmaps before tests to consolidate info the day before.
You can find lots of topic questions at Physics and Maths Tutor and if you run out, just ask your teacher for some!
If you need video help, I used Bicen Maths and MissEstruchBiology. They also have great summary videos.