Hey,
I am also going into Year 13, and am studying Maths and Further Maths. In terms of Maths, the best thing you can do is Practice as many different questions as possible. If there is any area of a topic you dont fully understand, what I have done in the past is grabbed friends, found a spare room with a massive supply of whiteboard pens (And of course a whiteboard) and got them to explain it through an example that they understand. Then after this, they would wipe off the example, and I would repeat the same question whilst explaining it how I would understand the question. If I got the question right, I would go another couple of questions on my own while my friends spectate my work (And making comments like friends do) and at any point I get stuck, we would talk about it until I understood the method.
If your friends also do not understand, then the wise choice would be setting up a time after school with your teacher and have them reteach you the topic.
My assumption is that you are sitting the new spec as you are only just going into year 13, and don't get me wrong, Maths is definitely one of the harder A Levels to take, but the reason I have found as to people not doing as well as they can is lack of motivation, and lack of talking to peers and teacher (I apologise if you harness a huge amount of motivation, and are always trying to improve through this method).
In A Nutshell, Revisit topics regularly, always attempt exam questions, and always ask peers/teachers.
If you have any further questions, feel free to ask me and I shall try my best to give you the required knowledge that you desire.