There are too types of riding; English and western. Now depending on the style you are doing, your attire will be different.
For english riding, you should purchase yourself a pair of breches (english riding pants) and a pair of padock boots at your local take shop. If you want to go really fancy with your foot wear, you can purchase, once again at your local tack shop, a pair of field boots. As for shirt, you can wear a simple polo shirt ( you can puirchase polo shirt at a tack shop aswell as normal clothing stores).
Western riding is more laid back and casual. You must simply wear a pair of jeans and a top of your choice. As for foot wear, I suggest once again a pair of padock boots or even the classic cowboy boots.

Have a wonderful and safe time!

If it’s an english riding camp then breeches are a good idea, with any kind of sporty-breatheable tops (or collared show shirts if its a fancy camp), and if it’s western jeans are good with t-shirts or button ups. Hope this helps!

Jeans and a shirt you don’t mind getting muddy.

what type of riding do you do?

i would wear breeches.
and a collared shirt.
a trainer really shouldnt complain at that:]

have fun!:D