comfortable shirt, very comfortable pants, and comfortable shoes. Those mean different things for different people so it's up to what you feel is comfortable.
I stated out riding in an arena in Germany (too many years ago now) and I usually went right after school. Riding in jeans, a cotton shirt, and tennis shoes. Over 50 years later the only things that have really changed are the pants (but I still ride in jeans since my favorite silk pants don't self clean after ever ride) and the shoes, but my favorite shoes for riding are still just (very ugly) glorified leather tennis shoes. I more often wear some of my military boots, but that's in case I end up in the woods and they offer better protection....and they are the second most comfortable footwear so it's not really much difference.
Wear what feels good since you want to be comfortable while riding.