Should I start with powered flight, or with a glider?
Depends on your preferences. Some prefer gliders, the 2m (6 foot wingspan) class is a good size large enough to be easy to fly but small enough for easy transport. Beginner gliders are lighter, fly slower and are more acceptable to the non-flying community than powered planes no noise or mess. Gliders are also cheaper (at least the trainers are) than powered planes no fuel, batteries, starter etc. to worry about. Electric Flight is silent and clean so finds greater acceptance from neighbours etc. at the flying field, although some people feel that electrics are not robust/easy enough for beginners. There is a little more paraphernalia - you may need spare battery packs, but you can fly from smaller fields. Power (with engines that use a fuel) will let you fly longer, and your model doesn't need to be as light as with electric (so it's likely to be easier to build); however, you may have to go to a field far from populated areas.
Whatever you pick, stay away from those sleek fast scale jobs! They look nice and fly hard, and those are the last things you want in your first plane. Stick to the trainers - they might not look as glamourous, but they will make you a much better pilot. And the most important point - no matter what else, try to find an instructor! This is the one thing that may make the difference between a rewarding experience and endless frustration. R/C pilots are friendly, and most will gladly teach you for free.

Are there any organisations etc. I can join for information?
Ask at your local hobby shop (Streamline Sales in St-Leonards-on-sea, East Susssex)or ask a club member. This is the best way to meet other pilots and find an instructor. Most pilots will more than glad to help you out. If you can find a club, for an instructor, choose someone who is smooth in flying his/her plane and that you get along with. Remember, the ones best at flying (hangar or otherwise) may not always be the best instructors.
For U.K. residents, an organisation well worth joining is B.M.F.A. They are the modellers' main voice where it matters they liaison with the Local Councils, the Civil Aviation Authority and other Officials. It is an affiliate of the SMAE and is the U.K`s aeromodeling representative of the Federation Aeronautique Internationale (FAI). Membership in the B.M.F.A also gets you £5,000,000 of liability insurance, without which most fields will not allow you to fly. You also need to be an B.M.F.A. member to participate in some contests. Besides, you also get a magazine, "BMFA NEWS" which is rather good in itself, and it keeps you informed about the state of the hobby. So JOIN the BMFA!!!

Radios

Since a trainer needs only 2 or 3 channels, should I get one of the cheap radio systems?
Don't bother with the cheap 2 or 3 channel sets get a 4-ch system. It will come with NiCad rechargeable batteries and (usually) 3 servos; this is the most popular and most cost-effective kind of system. You can put the main pitch control (elevator) and the main turning control (in this case the rudder) on one stick, which is how most people (and thus most instructors) fly. The cheaper systems come with the controls on separate sticks and you will have tough time finding someone willing to teach you with that setup. They also use non-rechargeable cells, which can get very expensive, and sometimes have corrosion problems at the terminals.
If you can afford it, a system that has a "buddy box" is a really good idea, and as most radios these days have this facility. This is an arrangement where the instructor's radio is hooked up to yours, and he just has to release a button on his radio to take over control, rather than wrestling the radio from your grip. If you do this, be aware that you need to get the same (or compatible) radio as your instructor.