Don't think of the RAF as a route to commercial flying; they want people who want primarily to do military flying and nothing else (and are prepared for the possibility of being killed as a consequence of that decision). It is only when their engagements are coming to an end that RAF pilots think about the commercial option. You will not get past the first CIO interview if you reveal that your ambition is to be a commercial pilot.
As for your actions now, your schoolwork must be your priority. During the long summer holidays, get yourself to a local flying club and offer to help out. You may find that they'll give you the occasional trip as well as a small amount of money for your efforts. When you can afford it, consider doing a PPL. However, research carefully what commercial sponsorships are around and what their terms are when you get to that stage, as some prefer their candidates not to have a PPL.
At some point you will need to consider the possibility that you may not be successful in your ambition (as many are not), and that therefore you'll have to earn a living another way. Consider carefully what other occupations might interest you, and make sure that you are in a position to go that way should the need arise.
Best of luck!
Scroggs