As said above, you need to seriously consider whether you want to fly or fly as a pilot and an officer in the RAF.
Bear in mind that thousands of people apply every year to become a pilot. Competition is very fierce. You will need to work very hard to get in. It sounds as if your academics are not strong at the moment, so go and get the qualifications to the best of your ability - the maths and science subjects are favoured. There will be lots of applicants that are extremely qualified, so to maximise your chances, you'll have to show strengths in other areas too. Your motivation for joining will be examined, so you need to prove that you have wanted to fly in the RAF for a long time. If you are not already, think about working with Air Cadets - maybe as a CI. You need to maximise your exposure to the RAF. Also try to get involved with projects/expeditions/adventure training etc.
As for the question about the chances of becoming a pilot once you have joined. If you have been selected, then the board has placed a certain confidence (on your ability, motivation and work ethic) that you will achieve it. That said, I know a lot of old friends who ended up getting chopped or left of their own accord. More so than I imagined at the start of training.