Not really fussed what I wear. Just as long as I remember to bring a change of clothes. Don't want to leave a session with a T-shirt covered in custard
Cargo shorts (preferably khaki or military-style, and a bit baggy in order to have space for the gunge inside the shorts), t-shirt, baseball cap, and sneakers with short socks might be a good option for me.
(It will be fun to start with a hoodie as well, but to me it is important to expose a significant amount of skin to the gunge for a more intense experience.)
I don't get to play nearly as much as I'd like, but my thoughts and fantasies typically involve trashing a like-minded guy stripped down to just his boxers or a classic white jock strap - or having him strip me down to just my boxers or a classic white jock and totally trash me.