Look here
http://www.royal-navy.mod.uk/server/show/nav.4994
The best place to start is obviously the careers office, they will have the latest info with regard to medical requirements, I am fairly sure that when you start aircrew training your eye sight has to be 20/20 without corrective lenses!
If your eye sight is acceptable you will need to pass aircrew selection, this is a bunch of mental agility and physical coordination tests, make sure your mental arithmetic is up to scratch, lots of speed, distance, time questions, they say that practising computer games can improve you coordination skills.
As well as aircrew tests you have to pass the AIB this is most peoples stumbling point< they are looking for Officers, so you must demonstrate, leadership skills, mental agility, intelligence, a knowledge of current affairs and all round sound character ( I really don't know why, I have met very few officers with any of these skills!!!!!!)
At the AIB you are required to carry out a multi-stage fitness test (bleep test) to your limit, so basically be as fit as you possibly can be, I would suggest at your age you need to be achieving around level 12 minimum or running a mile and a half in less than 9 minutes. RN fitness test gives you around 11 mins I think.
Hope this helps, but you really need to visit a careers office.