Well....
Fitness: The level of fitness that will be expected of you when you first join will not be particularly high. A basic level will see you through the first few weeks. A good indicator of whether you're fit enough would be to go for a 1.5 mile (2.4k) run. This is the standard fitness test that you're expected to pass with the required time towards the end of your time at Raleigh. If you're under 25 years old, you will have to complete it under 11.13. If you can get underneath this time before you start Raleigh then you will have no problems at all. It isn't hard tbh.
Pay: Pay isn't brilliant to start off with, it will soon improve though. I'm just a standard rating (ie - I havn't been promoted, I'm not an officer) and I'm earning just under 21k atm. If you get your head down and try to get promoted you can realistically expect to reach leading hand level with 5 years. The pay at this level will be at least 26k, rising to above 30k depending on time served. As an officer you can expect to earn a lot more than ratings who have served the same time. A leiutenant can expect to earn 30-40k easily. The real money takes a few years to kick in, this is to encourage people to stay in and not leave early (like me!).
Training period: Initial training for me was 8 weeks, although I believe this has now gone up to 9 weeks. I'm not sure how long it is for officers, it will be considerably longer though. After initial training (how to become a sailor, how to march etc etc) you will go off and learn your trade. As I mentioned, I'm an aircraft engineer. I spent 6 months at HMS Sultan in Portsmouth learning basics about aircraft, this is mainly the theory side of things and involves lots of books and tests. I was then drafted to my first squadron at RNAS Culdrose in Cornwall. I spent another 6-7 months learning to fix helicopters hands on. From joining the navy to becoming a fully qualified engineer took about 14 months. My trade requires more training than most others within the navy though. If you join as a chef/warfare specialist/weapons engineer etc you can expect to be fully qualified well within a year.
Perks: You will learn a trade, this may prove useful in a future capacity. You have job security. Pay is good if your decide to stay on for a decent length of time (5+ years). There is a very big social side to life in the military,
lots of nights out (may not be a perk if you're not a big drinker). You will make some close friends. You get to travel, not so much in my trade though. I have had a few good trips, I spents several months in the States and have been to quite a few countries around Europe. Loads of chances to play sports. If I'm being honest, there are loads of perks. The reason I'm leaving is because the military life is not for me. I find it too restrictive, I like to lead a life on my own terms. However, there will be a lot I will miss when I leave in 2 months.
Life Experiences: It may be a MASSIVE cliche, but life in the navy is what you make of it. There are a million chances to do things that you may never have experienced. I wenting skiiing in Bavaria last year and it didn't cost me a penny. A mate of mine went hiking in Peru for 3 weeks not so long ago. If you're half decent at a particular sport, you will get a chance to play competitively (and get loads of time off as a result). The big atraction for many is the chance to travel. You will go to lots of cool places, although a lot of the time you will only get to see the city where you're docked. You will meet loads of new people as well, some of them you'll become close friends with, some of them you will hate their guts (only natural when you're living, working, socialising with the same people all the time).
Entry Requirements: Depends on which trade you choose. It also depends on whether you want to join as an officer. My trade required 5 GCSEs A-C when I joined, I think this may have changed though. Most trades do not require any qualifications. Officers often need a degree, this can be in any subect though and it doesn't even have to be a good degree (2:2 would probably be enough). A lot of the guys I work with are more intelligent than the officers if I'm being honest
You may be required to take additional maths/aptitude tests if you're joining as an engineer. You also have to pass a medical and a couple of interviews. The interviews were more of a formal chat though, to make sure you're not a terrorist or something.