You have got every chance of doing well in GCSE! You seem to have realised that this is serious, so you can definitely get those grades up; make sure you believe in yourself!
History was my hardest subject, and I bought the specialist revision guide for the textbook we had at school (i did OCR modern world history) and it was so helpful as it taught me everything I needed to know. I would also use spider diagrams (not just for history) to condense all of the information and note down key dates in timelines (eg for the events leading to the first world war)
I also cannot stress the use of past papers for science/maths enough; they are the best practice you can have.
Good luck!