If you think about what an exam's doing, then it's easier to see why certain techniques are more/less effective. It's essentially a problem-solving exercise where your deductive reasoning skills and application of principles are being tested under timed, controlled conditions. The topics that come up can be more or less random (within certain boundaries), so it tests your broader understanding of the subject.
If you need good exam results, you need to develop a good working memory - which works in stages of attention, encoding, and retrieval. Good exam technique also means better time efficiency and making points in a more succinct, less beating-around-the-bush way.
The internet is such a massive resource that you can teach yourself almost anything, as long as you learn how to filter out the misinformation. There are a lot of mathematics/science channels on YouTube, including ExamSolutions for A-level mathematics and KhansAcademy for biology.
The very first thing is to do is to be organised - at the very minimum to get by, you need to have a copy of your syllabus and the relevant notes/revision-books for your topic.
During classes and when reading, try to elaborate - think very deeply about your subject and ask yourself "why is that?" or "how does this work, in terms of its mechanisms?" and attempt to come up with the best possible answer you can. If it helps, explain your topic by analogy or break it down piece by piece and explain each step.
If you want to go one step further as a fairly advanced student, it helps a lot if you can understand/derive the principles behind any formulae or explanation you've been given. These skills are more essential for University level students, but it is certainly helpful for A-level students if you're able to do it.
It doesn't matter if you know very little of the subject at the start - try to teach someone else the topic anyway. It helps because it forces you to break down the content and understand/explain each point, instead of rote-based techniques used in GCSE.
If you've completed a few past papers (closed-book), you should have been able to identify some topics you have trouble with. Taking good notes by summarizing and explaining the key points in the book, you should encode and retain the information by completing some exam questions on the topic and improving on your answers until you have a strong understanding.
It's also really important to avoid *bad* learning techniques, it's quite easy to trick yourself into thinking you've learned a subject.
The worst methods of learning involve opening a text book and highlighting it, or re-reading. It creates the false sense that you've learned/understood a topic, but what you've really done is created recognition where you can't think of it on your own but it suddenly comes back to you when given a blatant reminder - something you won't have in exams most of the time.
In an exam, you need to know what information you have and what to do with it. To improve time efficiency, if you're not doing a literature subject like politics or history then making bullet points and underlining key terms used in mark schemes help on identifying mark allocation and how much time you should be spending on each question.
The structure of your answers should be succinct, easy to follow, and logical. Literature based courses have a particular essay structure to follow - e.g. explanation of terms, introduction, arguments for and against, summary, and conclusion. It is essential to stick to this structure.
It takes about a hundred hours to learn any particular module - less if you're efficient and pick up new concepts easily. It is essential to have stronger core knowledge to avoid falling behind at later stages, so develop that first before moving on.
The first closed-book papers you complete may fall within the 50-60% range. That's perfectly fine, even strong students do. The aim is to improve by 3-5% on each paper, until you're comfortably averaging above 85% raw marks (usually an A grade).
It's possible to keep going further and pushing your grade closer to 100% but it's not time efficient - just move on to the next subject until you're doing well on almost everything. During your exams, you should be well fed and rested so you're able to focus.
If you follow this advice, it shouldn't be *too* difficult to get a B/A grade(s), and if you apply yourself hard enough and make few mistakes, A*s are certainly within reach.