Well my advice would be, for most subjects except English: really try identify the areas in which you are weak in for each subject and compile this into a list. To do this you may want to do a practice paper and mark it yourself or get a peer or teacher to mark. Wherever you get wrong ensure to note this into a list. If you can grab past papers from your school from your teacher, then all the better.
Once you think your list is sufficient enough with all aspects that you can improve on, then actually take time to review in the textbooks, everything you need to. You can use whatever resources available to you, but ensure to understand the topics. Regularly do past papers to try to identify further areas you are struggling in and to get a feel for the style of exams and questions that could be asked.
Next important thing is exam technique and understanding the paper. Now exam technique can be learnt, and this can be different for different exams. E.g. for a subject like Physics, drawing diagrams and headings are often a good technique. Understanding the paper means to understand the marking criteria and how it will be marked. So I'd look through exemplars, marking criteria from past papers to actually understand what the marker is looking for so you can be awarded marks. Looking at BOSTES key words is a pretty basic idea in terms of understanding the paper and what the marker wants - but there may be more ways to understand a paper and how it's going to be marked depending on the subject.
Practicing these two things should help in order to do well, and doing this enough should make you more confident as well as making exams much easier in terms of gaining marks and getting better results. Doing these more and more should a) give you a greater understanding of all content that can be tested and b) make it clearer to understand what the markers actually want and make it easier for you to do it, rather than blindly trying to gain marks.
There are many other things that can help you, such as peer marking, resource sharing, tutoring, in-class questions as well as paying attention, how to stay motivated etc. etc. but the above advice should help you in achieving the extra marks, in cases where you may not be sure why you are getting lower results. Also English is slightly different, and a different approach should be taken to do well in it, so if that is your main concern I'd advise getting some other advice (I can try provide some from personal experience) for this particular subject.
And remember, if you find that you are trying this and by end of term 1 or 2 nothing is happening, or you know this advice isn't relevant to you, you don't have to be foolish about it and keep doing this. There may be other underlying factors for your results that I'm unaware of and this may be the cause of your lower marks. I'd step back, and try to identify the underlying reason for your lower marks and try to fix this. This is not some magical guide that will give you success by following it word for word. But hopefully this is some good advice that will help you.
