Just starting A-Level Maths, for 2020.Watch
As well as that, I really suggest actually understanding what's behind the concepts (e.g how the formula for differentiation came about, how the formulas for calculating probabilities, variance, and mean came about). "Khan academy" is a website which is amazing for this! It really helped me understand things like trigonometric identities, and calculus.
And finally, something you will probably get a lot, try and do exam practice asap! This doesn't mean you need to be doing exam papers two months into your course. Rather, every time you finish a topic, search up online (or buy a workbook) good exam questions based on that topic. Exam technique is what will be the difference between an A and an A*.
If you're thinking of maths for uni (or some sort of STEM subject for uni), consider doing some UKMT olympiad questions sometime in year 12. A lot of admission tests have questions which use AS level maths content, but the questions force you to think outside the box, so students that usually get full marks in an AS paper may struggle a lot with these types of questions. If you're not interested in admission tests, no matter, doing these types of questions can help you think differently and can help do your Alevel maths exams much quicker and easier!