You don't really have to wear anything in particular, but just make sure you wash your hair! I say this because my friends and I frequent this party that has a blacklight room and a guy there had obviously not bothered to shower that day and his dandruff was incredibly obvious. I felt kinda bad for him because no girl wanted to dance with him, haha. I'm sure you don't have to worry about this, but it was pretty funny.
take UV paint or the liquid from glowsticks and just splatter it across the white tshirt. it gets messy but it looks awesome. if you manage to get the glowstick cut open, be careful because there is fiberglass in it. so I suggest splattering it and then brushing off the front of it once its dry. have fun!
If you bring a higlighter you can write on other people! And if you put any kind of laundry detergent on anyhting it will glow too! Highlights in your hear and fake nails all glow too so be careful! Have fun!