We would suggest you wear jodhpurs/jeans/leggings to protect legs from chafing against the saddle, and close-toed shoes with a small heel to keep feet from slipping out of the stirrups eg boots rather than trainers.
In cold weather, we advise to wear layers of clothing to keep warm.
In warmer weather, we advise you to apply sun tan lotion before your lesson/hack to avoid burning.