I would say to work hard at school in all your subjects!
Maths and English are both important in science.
Beyond that I would say chose scientific GCSEs and A-levels and keep your interest in science up by looking at videos online and reading about science (the horrible science books are great for this – I loved them!)
I think that science needs to be fun! In my opinion, the key to being a good scientist is to find an area of science that you are really interested in and can get really excited about. Once you have found something that really interests you, you will just want to keep learning more and more about it – until you become an expert!