First, make sure you take the correct A-levels!
Mathematics and Physics are absolutely essential. Chemistry is possibly nearly as important.
Then you'll need three years of study at university to get a bachelor's degree in a relevant subject. You'll need to be a high achiever (getting a 'First' or '"2:1" degree) in order to be accepted for a further year's study for your master's degree. (There are a few 4 year courses, following on from A-levels, which aim straight for a master's degree). Then you'll probably need to spend a further year (or possibly two) at university working on research to obtain your doctorate.