Edit: For those answering re: Halter and Showmanship classes, we don't have those in the UK!
Ringcraft - All in on the left rein and you will keep going round in circles until the steward asks you to stop and you will then take it turns to run to the back of the line and then asked to walk round again and the judge will pull in his provisional placings then one by one you will all present your pony to the judge ie standing square and making him look interested (rattle of a tic tac box in the pocket always does the trick if hes not spooky, dont give him the tic tac though until you are back in your line up) you will then walk away from the judge and then turn your pony away from you and trot back to your place in the line. Once everyone has presented to the judge you will then be called in for final placings. Once you have received your rossie (and everyone else) you will be asked to do 1 lap of honour which is done in trot and then you will exit the ring.
Your horse will need to be shown in a bridle that compliments his head along with a suitable bit. Make sure though that bit does not drown his head as I have seen this way to often! Depending on what class you enter, you may show 'in natural state' (ie; not plaited, as in mountain and moorland classes), or plaited and polished. Check the rules for the show you wish to enter.
Your attire - I would wear navy, or light coloured. trousers as jodphurs are for under 14s or 16's. A nice colour shirt, say pale blue or green with a tie that compliments, with either a navy velvet hat or a trilby type hat pref brown in colour or whatever compliments your outfit but keep it simple (check shows rules as to hats) brown joddie boots which are comfortable and brown gloves/cane.
There's lots of information on showing in the UK on this site:
http://www.theshowring.co.uk/
And some great advise here:
http://www.h-10.co.uk/horse-advice-winning-in-hand.asp
Good luck, and don't forget the most important thing is to enjoy the day!