An equitation class is judged on the rider. Their position over and in between fences- are your heels down, leg quiet, shoulders back, head up, etc. You want the course to look smooth and not rushed. The rider's ability to bring the horse to a correct distance and how they handle the jump course that is given are also judged. Generally, equitation turnout is similar to hunters- show jacket and shirt, belt/breeches, tall boots, black gloves, hair up in the helmet, and (at the rated shows) braided mane and tail.
If it is a schooling show, you should be fine with a nice polo. Anything else and I would wear a jacket if you have one. Good luck and have fun!