I suggest you first focus on fully understanding EVERYTHING in lessons - if at any point you are confused, spend some extra time going over your notes/ go along to a lunchtime workshop session if your school runs them/ ask your teacher for help/ look up more information in a textbook until you really understand it.
At the end of each topic write up your class notes into more concise notes - this will help to consolidate the information in your mind, revise for any end-of-topic tests, and give you short revision notes for exam time. I suggest reading through these again over the Easter holidays - and if necessary re-writing them.
I also found it helpful to create a revision quiz using flash-cards, and a couple of us did it at lunchtimes and on the bus
It's quite a fun and relaxed way to revise during random short sections of free time. Another way of making revision a bit more fun (and hence, more memorable) is to create colourful posters for each topic and sticking them on your bedroom wall (you can even take it a step further and coat your entire house in posters - so you can revise by reading the posters in the shower each morning...).
Over Easter and the following term, focus on past papers - literally do ALL the past papers from the last 10 years or so. I found it helpful to create a spreadsheet with all the past papers in a grid - when I completed a past paper I wrote my score in and colour coded it depending on how well I had done. I worked from the older papers to the newest, so could easily see my improvement across the spreadsheet as well as ensuring I am practicing the most recent past papers closer to the exam. When you do past papers, try to complete them in the time limit and do your usual checks afterwards. Make sure you mark them ABSOLUTELY according to the mark scheme - don't give yourself marks for being 'almost right'. If you notice particular questions coming up frequently, it might be worth writing example answers.
Just before the exams (say a couple of days before) re-read your revision notes again and make separate notes on stuff you didn't know off-by-heart, then focus on just memorising these (hopefully just a few) things before the exam. Some people find it helpful to read through their completed past papers too.
This might sound like a lot of work - but start preparing and revising early and actually it is very manageable
Try and give yourself plenty of short revision breaks, and work in a quiet studious environment so you can really concentrate on getting work done. I found turning my computer/ phone off helpful too - and often went to the library to avoid distractions