First off, you should be mega proud for getting back into this. I got a 7 at GCSE and was pretty chuffed with that (equivalent of an old A grade) and here's my advice:
1. Practice papers. The only decider of your grade is going to be an exam, so you need to get good at doing them.
2. Don't cheat yourself. If you find something boring or tedious don't avoid it. That's where you're going to lose your marks. Practice it more until you enjoy it. If you didn't know something in a practice paper you're going to spend the next week committing yourself to doing only that.
3. YouTube. Watch as many different videos of the same mathmatical concept that you can on things you don't undertsand. You'll get it eventually.
4. Flashcards. Use these for remembering formulas, equations, and definitions, and especially for the circle theorms.
5. CGP book. Buy the revision guides and as many practice books you can afford. You're going to do these before you get started with your practice papers.
The thing is about maths is if you practice anything enough times it all actually becomes really easy.
I wish you the best and good luck with everthing. Reply for any more questions!