Before you consider self-studying maths, make sure you understand the content you have covered in class well.
From there, you should probably start pre-learning content that your class will be covering in the near future, so essential accelerate their order. This way, what you gain from self-studying will be applicable to your upcoming tests.
In terms of actually studying, use a solid textbook (Like Cambridge) and watch Eddie Woo videos on the relevant topics to gain a deeper understanding.
https://www.youtube.com/channel/UCq0EGvLTyy-LLT1oUSO_0FQ
Best of luck