I achieved an A* this year. It’s all about doing loads of practice really. When you’re in Year 13, I recommend buying the Edexcel practice books that go alongside the main textbook you’ll probably use in school.
In terms of free stuff, there are tonnes of questions on madasmaths website, and some practice papers too. Bicen maths on YouTube is good for statistics, I also found Nancypi channel useful (American, but explains things clearly),
Good luck!