Clothing - I would suggest jacket & tie as if you were riding & light couloured trousers Fawn or beige not white.  Foot wear - something you can run safely in jodphur /yard boots or even trainers but mind your toes.  You should also wear a hat, probably a riding hat with hairnet if necessary particularly if showing a youngster, some wear a 'stockman's' hat. 

Depending on the age & type of your horse an inhand bridle with or without bit or an ordinary bridle will do make sure you can control him in what ever you use.  Plaiting will  depend on the type of you horse, if you are not sure how to- practice.

From the judges piont of view first impressions are very important  be tidy, have a clean horse, and smile, watch other more experienced compeitiors to see how thay present themselves