a twin trap boat is likely to be a handful for a beginner.
You've never seen a Hobie Twixxy do you? Or a Dart 16? Both are twin trapeze, none can really be considered an "handful"
What kind of Hobie are we talking about Tim?
To answer the original question. Get yourself a harness designed for sailing. Avoid the windsurfer harnesses. I'm sure there is a dinghy store somewhere on the island, go there and try them. All the serious shops have a loop attached to the ceiling where you can try your harness for fit. The cheap crewsaver harness that you can find everywhere in the UK is good enough.
The best way to try out trapezing for the first time is to be with someone who can show you. Just approach the other cat sailors at your club/beach and ask them. They'll show you. Take someone with you in all cases anyway so you can practice getting in and out without having anything else to worry about (tiller, main sheet...).
The principle is simple. Sit on the hull, get the loop of the trapeze in the hook of your harness. If the trapeze is designed properly it will stay in place in the hook and not drop of. Push with one hand, get your butt outside the hull, putting your weight on the trapeze line. Then put a feet on the hull and push, you will end up standing up sideways on the side of the hull. That's all there is to it.
Start with nothing, then try with a sheet in hand, then with the tiller, then with the tiller and a sheet. Advanced mode is my Nacra 5.2 where you end up with tiller + sheet + downhaul + mast rotation + jib sheet and jib barberhauler
Unless you're a pro here is a list of don't [Those are don't for beginners, there are advanced techniques where those don't do not apply, I know...]:
* Do not stand up to put the loop on or off. Stay seated. Make sure the loop is low enough so you can hook it up while seated.
* Do not grab the handle. Leave it alone. If you pull on it that's when your hook is going to get undone. Trust the trapeze line. The handle is only useful for one thing: coming back on board when you are trapezing low. You never need it to go out and you never need it to stay out.
* DO NOT GRAB THE HANDLE. That's a bad habit.
