If you wanted to do dressage you should definitely look up reining on the western side, its almost the same thing, only faster and in a western saddle, with different moves lol
As for attire, jeans, boots, western shirt, hat that's pretty much it, but then again I don't show, but I think that's pretty much the basic, I know a few people on her that do western pleasure...but I can't for the life of me remember their names....anyway
Good luck!!