The site that Rev linked to above is a good starting point - I too am applying to the RAF and have read through that site before. It'll only take you 30 minutes or so to read through everything on the site and it'll give you a great idea of what you'll need to do when you get to OASC.
Try and talk to some pilots too, get in contact with your nearest base and see if you can arrange a visit etc. Not only will they be able to give you advice about getting in, but they'll be able to tell you what it's like once you are actually in!
From what I can ascertain, motivation is a big part of getting accepted - if you can prove you've put in the legwork then they'll be much more impressed. Don't aim to do enough to "pass", aim to excel. Training is tough (even tougher if you want to fly FJ) and they need to know you won't fold under the pressure, and can push yourself to succeed.
This info comes from someone as inexperienced and new to it all as yourself however, so don't take what i say as gospel! This is just from what I have picked up from talking to ex/current RAF and my own researching. Unfortunately I am only really going to get one stab at this (I am so old my bones are creaking - 22, coming on 23!) so I am doing everything I can to give myself the best chance - I would suggest you treat it as though you only have the one opportunity too, it's a great motivation factor!