You should wear a blouse with a rat catch and stock pin. Beige breeches, tall field boots, a black, blue, or dark green show jacket, and an approved helmet. Oh, and black gloves, and don't forget a belt.
Black or Navy Blue show jackets are very acceptable, and stay in style...where green, brown and grey can go in and out of style depending on where you are and when.
Beige/Tan/Greeny Beige are also all perfectly acceptable breeches.
Show shirt that fits properly. White of course will always be in, but pastels are also considered "in" right now as well. Monogramming on the collar is really popular.
Field Boots. You can use half chaps and paddock boots for showing at lower levels.
A nice belt....in case it is hot during the summer and jackets are excused, as it makes you look more put together.
Black Gloves. They help your hand blend in with your jacket and be less noticable. Also nice if you have a darker horse, and you need to do a quick correction, it is not as easily visible. (I keep a pair specifically for showing so that they are not dirty/grungy as I would forget to wash my regular everyday ones.)
You might want to invest in a garment bag so that you can hang your jacket and show shirt in when travelling so they stay neat and clean. That way you can also leave your hairnets and spare bobby pins in there as well just in case.