Further to the last post, I guess it might just be easier if I do the whole "Welcome to St John" spiel right away, rather answering specific questions.
I joined St John at university, but I know a number of sixth formers who have joined as a method of getting experience for medicine applications. It's a great charity, and can get you a lot of practical experience, but that being said, the organisation isn't always viewed positively by NHS trusts, and you can encounter a lot of skepticism.
Joining St John, as others have alluded to, can take a while depending on where you live. You essentially have to fill out a application form, and then undergo basic first aid training in Emergency Life Support, Family First Aid, Manual Handling Principles and a course on how to fill out our legal paperwork for each patient. I teach some of these courses to new members myself, and they essentially cover the basic situations that you might encounter in the workplace, at home, or while providing first aid cover at events.
Once you have all the essential paperwork out the way and are trained up, you are free to go out and cover events. This can range from music concerts to village fetes, sports matches to the Marathon, and rumour has it we are coving the Olympics. On a typical duty, you can encounter anything. I've treated people who have had strokes, bad cuts, people being sick, unconscious people - you name it, it can happen.
The more you train, the more qualified you can get. I am now a Patient Transport Attendant, which allows me to transfer patients to and from hospital. You can also become an Emergency Transport Attendant, and these can cover 999 calls for the NHS in some areas. You can learn to use oxygen, AEDs and a number of other skills depending on where you live.
That's the basics anyway. Let me know if you want to know anything specific