1) As for the halter class, it says "Showmanship at halter(English or Western)" Do you go in in a halter even if you(the rider) are in english clothing? Is a clean leather halter with a nameplate acceptable? Any certain leadrope I might need?
Grooming and Showmanship with English attire typically allows either a halter or a bridle. If in doubt, check the rulebook for whatever organization is operating the show, or ask when you register. If you use a halter, it should be plain leather with no nameplate or silver bling, and should fit the horse well. Your lead should be a matching leather strap with a chain the same color as the hardware on the halter. The chain is run the same as in Western halter.
Most English G&S classes I see or compete in have a pretty even mix of entries in halters and in bridles.
2) What do you(the rider) wear for the western pleasure?
Show pants, chaps, belt and boots (If these are all the same color, the overall picture looks really professional), Show shirt, and cowboy hat or riding helmet. Your show shirt should coordinate with your saddle pad. You can look up some pics of WP riders to get an idea of the styles of show shirts that are "In" now - lots of glitter, big collars and cuffs. Choose a color that complements your horse. Hobby Horse Clothing Co. - Description
3) What exactly do you do in the Western Horsemanship class? From what I've been told it's like a pattern of cones,etc.
Western Horsemanship operates much like an equitation class, with a pattern to test the rider's ability to control the horse, etc. The focus is on the rider, as opposed to pleasure, which focuses on the horse. Your pattern will be posted ahead of time for you to memorize before you enter the arena. Cones will be placed in the arena to mark where a given movement needs to occur. Stay on the side of the cone that is shown on the written pattern, and make your transitions as prompt as possible (for example: if the pattern calls for a left lead lope at cone B, you should be picking up the canter as the horse's shoulder passes the cone).The pattern could be about anything, so work especially on your transitions, halts, backing, and precise circles, loops, and straight lines.
Good luck, and enjoy your state fair!!