Are you showing at local level?  If so I wouldn't panic too much, just turn yourself out neatly.  Ideally a tweed jacket (but black will do if you have one and are short of cash!) shirt and tie - a coloured tie always looks nice.  Trousers should be the opposite colour to the horses legs i.e black trousers for light greys and horses with white stocking and cream/beige for bays or dark greys that have black legs.  This is to ensure the judge can clearly see your horse move.  Shoes don't really make a difference, just make sure they are comfortable and you can run in them on grass!! I've seen people with trainers, dealer boots etc... and as for wearing a hat, surprisingly a lot of people don't, asses the situation, read the rules, and make a judgement yourself.   Ideally the judge should be marking the horse and not your appearance, so be sensible with what you wear, and look around at others for ideas next time!  Enjoy!

I'm having this dilemma as well as I'm about to show my yearling and have never done any inhand stuff before!  Apparently it is recommended to wear trousers the opposite colour to the horse's legs, i.e. if the horse has white legs then wear dark trousers.  I'm going to wear a velvet riding hat too as with youngstock you never know, and I've also been told that waistcoats are optional.  I have read that brown footwear is recommended but as the hat will be black I thought the brown would be too severe a contrast so I think I'll wear black johdpur boots with black leather gloves, cream johds and my tweed with the hat.  Hope that will be acceptable after all they're meant to judge the horse not the person handling it, as long as you're smart and clean then I think it should be ok!