Wear something non-restricting and breathable. You will be sweating and will occasionally roll around on the floor (sometimes even on purpose). Sports tights and tank tops are popular options.
For your feet
A great pair of socks is your best protection against blisters. High ankle running socks, hiking socks, and the super cute socks from Sock It To Me, all work nicely, just make sure the socks sit above the ankle of your skate to avoid rubbing.
For your legs
There’s two goals for your legs. The first is protection, you want to avoid rink rash if you accidentally slide on your side so covering your thighs can be helpful. The second is movement, you don’t want your cross-overs to be limited by ill-fitting pants. Sports tights and leggings work well. Many men often wear long, comfortable sports shorts. But anything that covers your thigh and allows for plenty of movement will work nicely.
For your top
Nearly any type of shirt will work. Got a t-shirt repping your favorite team, a tank top that shows off your guns, or a shirt with the sleeves ripped off hulk-hogan style…you’re all set. Just be prepared to sweat in it. Also, for women, get a good bra that fits properly and supports you, likely any sports bra that works for you will work for derby.