As Baldrickswife says your pony would be too big for affiliated M&M Lead Rein classes - maximum height is 122cm (12 hands). He would be fine once you get into First Riddens (if 128cm) although a Section C would tend to look quite big compared to the normal ponies for those classes - As, (possibly Bs), Dartmoors and shetlands.
For local shows you'd just need to read the schedule to see if there is a height restriction.
You'd try and both wear tweed - most people have matching at affiliated shows but that is not necessary at all when you start off.
Brown tack and a plain browband for a M&M (sometimes referred to as a Heritage in schedules).
You'd have a leather leading rein which you attach to the ponies noseband. Ideally you'd carry a show cane or short simple whip if that is all you have.
Simple shoes with minimal heels that you can run in would be order of the day for this time of year. I know that lots of ladies wear boots in Spring and Autumn.
In the class itself you will all walk around on the left rein to start with. A steward will then stop you and ask you to trot to the back of the class down the long side in front of the judge. Stop before you get to the last pony and both bow.
You will be then be lined up (perhaps in order but not always) and expected to do a short show. This will normally consist of standing up for the judge and walking away with a trot figure of 8 and nice halt and joint bow.
Ideally the pony will be going on the bit and the child doing the steering but to be honest just go out and have fun !! Let us know how you get on..