For combined science i recommend watching videos by freesciencelessons and cognito on youtube, they have really useful videos and to complete as many past paper questions as possible and learning from the mistakes. You should also practise completing full papers under timed conditions to get good exam technique.
For maths you can watch videos by the gcse math tutor and cognito who has good videos explaining maths, then you got to keep practising questions.
For history, you should practise your exam technique by completing past paper questions and ask your history teacher to mark it. To learn content you could try creating timelines explaining events and its impact as well as blurting.