Good, thick clothing that you don't mind getting dirty or damaged. Minimise your exposed skin for two reasons: 1. Getting shot can really , really hurt even through clothing. 2. If it's an outdoor place, you might be crawling through the undergrowth. If it's an indoor place, they tend to be roughly made from plywood which can be abrasive on your skin.
Good footwear for running around in, proper boots (not Uggs!) are ideal.
Also see if you can find a pair of old gloves to protect your hands. The paintball place will supply you with a face mask and googles. Oh, and tie back your hair.
Wear clothes that cover your whole body that will keep you cool since it's still hot out. You want to be comfortable and mobile for sure. You're going to get bruises no matter what you do or wear. It'll be the most fun you had getting hurt.
Long sleeved shirts and long pants that you don't mind throwing away. Does the paint-ball hits hurt? yes very, but only direct hits at relatively close range (about 4-6 meter). Most of the time the pellets only graze you or are shot from too far away to cause too much pain. They will still sting slightly however. *DO be prepared to suffer a few bruises. Especially around your legs and abdomen area. As those are the areas most likely to be hit.
Wear some old jeans and a hoodie. The paint in paontball doesn't stain, it comes out easy. A hoodie is good because you can keep your head and hsir safe. Wear something comfortable. And by the way, don't be a rude person who shoots point blank. Just say surrender. Have fun paintballing! :D
Thick jeans, maybe even something thinner under the jeans to reduce the impact. If you wear a shirt or two and a hoody you will be fine as it will absorb most of the impact. You will be shot in the fingers and that is the most sensitive spot other than maybe your collar bone. When firing keep your chin tucked in with your marker to prevent that from happening. Best of luck to you. It's really fun!
Wear darker coloured clothes: Green, brown, black. Hell you could go to a military surplus store and buy a camouflage jacket. Also if you got boots you don't particularly care about, wear em. Also if they don't provide face masks, buy one, unless you like getting hit in the face with ball filled with paint going some 300 fps.
Pants and preferably a long sleeve. Also be prepared for the first time one of those things hits you. The first time i got shot a 12-15 year old kid came around a corner while i was yelling at a friend to cover me and shot me right in the ribs from about 10 ft away. It bruised up pretty bad.
long sleeves, long pants, and boots you do not mind getting dirty. and make sure you have on a face guard. Darker earth tones and black are good for blending in well, if you want a slight edge and camo is good if you will be in the woods.