Hi there,
I was in your situation only a couple of years ago. The best way, at the moment (in my opinion) to get into the RAF as a pilot is to apply for a bursary, but tell them that you will accept a Direct Entrant (DE) slot if offered one. If you do not get in after A-levels, and don't plan to go to uni, you will lose out to people your age who go off and do a decent degree, and complete EFT at a UAS (which happens to be the best pi$$ up club in the world!). I eventually decided on uni, and have just sent off my account details for my bursary!
As for the grades, you need C's, that's the magic letter. Nothing more, but certainly nothing less. Just work hard, and you'll pull through, I promise.
All the above advice is spot on, if you follow it, you will have a good chance.
PM me if you need any more help.
TMA