If following the UK system
-Study through A-levels, specialising in Maths, Physics and one other subject, preferably another science
-Find a university that offers a course specifically on Aeronautics, otherwise chose one which has close ties with aerospace companies and study Mechanical Engineering as this will cover most of the topics required of aeronautical engineering.
It is possible to study a BTEC instead of A-Levels but this will put you at a disadvantage. For students not in the UK, I would assume the system is very similar. (A-levels being the grades you study for before university)