Yay for you! It's fun to try new things, anyway! This might be a book, I love explaining classes...
Well, with Western, at least around here, Black jeans are what we wear. You should have a belt, and no, you don't really need the show shirts, just a western button up should be fine. You need a cowboy hat or helmet. Cowboy boots, too, though you could probably wear your english boots if you wore your jeans over them.
Um, I think that a stock saddle would work... But if you could borrow a western saddle, that would probably be better.
Western Pleasure... Let me link you. Western pleasure - Wikipedia, the free encyclopedia
Basically you go around the ring, starting with right shoulder on the rail, and do the walk, jog, lope. Then they reverse you and do the same on the other side. Then you usually line up in the center and the judge asks you to back your horse. LOOSE REINS. That's important.
Western Horsemanship is a pattern class. They'll have the patterns posted before the class. You'll probably have to do a lead change, a pivot, a little jogging, a backup, and a stop. Not too complicated. If I were you I would start with the easiest class, which I think, in your case, would be Youth, then Beginner, then Amateur. ((Someone, please, correct me if I'm wrong...))
Western is pretty much all on a loose rein, so neck reining would be great. =)
Trail is a course of obstacles that you might encounter on a trail, which usually include a gate, a bridge, w/t/l over poles, backing an L, a mailbox, and sometimes a water box. Not too complicated. Again, they'll give you a pattern. NEVER NEVER NEVER touch your horse or your reins with your other hand in trail. That's a DQ.
Ummm...I think you should do it. It would be a good experience, at the very least, and I think it's fun. As long as you are comfortable with trying something new, I'd say go ahead! I don't do much Pleasure, but I think Trail and HOrsemanship would be fun for you. Pleasure probably would too, but it's kind of foreign to me and I think it might be hard for you to adjust from flatwork to pleasure ha. Western Horsemanship is just like english flatwork, only in a western saddle, doing western things, if that helps. (I think... again, please correct me if I'm wrong. I'm not completely sure on what flatwork is.)
Have fun and don't hesitate to PM or ask any other questions!