I did my GCSE's in 2012, and I'm not sure what GCSE maths exams are like now because I heard they were making changes, but I highly recommend that you google 'hegartymaths' if you are someone who doesn't find maths easy. I thought I was never going to get my C grade, but using his videos and resources really helped me.
I also found it helpful to make myself short powerpoint presentations, for different topics, that I could talk through with myself. I not only enjoyed the creative side of making a powerpoint, but I also found that I memorised and understood the information better by saying it out loud and making it stand out, rather than reading some dull notes that I made in class.