GCSE Edexcel Computer Science 2018Watch
If you're struggling to the point where you need to go right back to the basics, have a look at these videos. There's about 6 hours of videos on here and some notes/exercises, but he pretty much starts from 0 knowledge and explains a few topics in very simple terms. He's pretty interesting and entertaining as far as CS teachers go, so a bit more fun to watch than most CS videos,
Have a look at the Computer Science pages on BBC Bitesize too, and try the tests on there, most of it should be relevant to you:
If you're having trouble with Python syntax or programming problems, try working through these online Python lessons (It's free - you can ignore the paid-for stuff..)
Similarly, if you need to work on SQL (Assuming you need to do SQL for your course?) - try these:
It's also a good idea to google for past papers and try using those to make sure you're comfortable with the kinds of questions they ask, but also to have more to practice and learn with, especially the types of problem-solving questions related to explaining the output of some code, or the types of questions where you're asked to analyse a problem.
When it comes to trying to learn terminology, try writing things down in your own words from memory without looking at the text book and then check your own definition against the one in the book to see whether it matches or whether you missed something.
Otherwise, a lot of computer science is about understanding and problem solving, as opposed to just trying to memorise things and cram knowledge, so really it just takes as long as it takes (The same kind of deal as learning maths really). The more time you can put in each week to practice this stuff, the easier it will be to solve those same types of problems in an exam.