If you decide you are going to go the modular route then I'd say the sooner you start the better!
Certainly now would be a good time to start thinking about how you are going to fund your training. Unfortunately you will need a minimum of around £60'000 to gain the ATPL qualifications. There's loads of info about it all over the place on here, have a search.
Your first port of call, before you commit huge amounts of money should be a CAA medical. When you've got that in place you can start to seriously look at the funding routes that are available.
Meanwhile work hard at school and get good GCSE and A Level results. If nothing else the discipline will help you when you come to sit the ATPL exams!
Good luck.
Matt