Hi!!
There isn't really one amazing system that works for everyone - I know it sounds annoying being told this, but you need to use whatever works for you. At my school, we are told to take notes in lessons then copy them up neatly and onto one page outside of school, to avoid the problem of messy handwriting and very little of a page being used. This takes time, but the copying out process is a great time to check that you know and understand everything that you copied down in the lesson! That way, anything you don't understand you can look up or ask the teacher about. You can also look up extra facts about anything you are interested in - this isn't essential, but it keeps you interested in the subject and may very well help you scrape extra marks in coursework or exams.
Outside of school, revise around the subject that you were covering in that subject lesson before, OR the subject that you were set to research for homework. Whichever you prefer. Just go over everything, answer questions on it, read articles online relating to the subject... anything and everything, it all helps. If you're really stuck on what to revise, ask your teacher what area of your work needs improving and then work on that! They may also be able to recommend websites related to the subjects you are studying for you to browse for your revision.
Good luck - we've all got the same problems at the moment, so you're not alone!!