Make sure you do really well on your GCSEs. you will need at least a C in maths and english. and it won't do you any harm to learn a language to a very high level as well, as this is desirable for most airlines.
you can't apply for most charter airlines until you are 18 and some of the scheduled the min age is 19, some 21. my advice would be after school to go out to work and get some face-to-face experience in customer services as airlines look for experience over qualifications mostly.
good luck!