I've said this a few times before, but just go for it and don't let anyone put you off.
Join cadets, get good GCSE results (they don't have to be amazing, but you will need good maths and English), do the A-levels that interest you (not the ones you think the RAF want you to do) do a degree if you fancy it (I did and loved it, but it's not essential). Apply when you're ready for it and work hard (OASC is not as scary as everyone makes out!).
Moreover, don't get put off by any of the whinging Jessies that spout off on this site and good luck.
I saw Top Gun (required watching for any wannabee pilot!!!) when I was 12 and 18 years later I'm loving my job. I didn't get to fly the Tomcat of my dreams, but I got the next best thing (honest!!).
BV