If you're a girl, then shorts (the shorter the better; you will get HOT), a light colored shirt/crop top/bikini top (black isn't a good idea. Trust me), and Converse, Vans, or creepers. Flip-flops and Toms aren't good for walking. Kellin Quinn wears Toms because he isn't walking around all day. I wouldn't wear boots either. A skirt is fine, but you might want to wear spanx underneath. Dressing like a hippie is fine if you want. It's more of a punk rock concert, so you'd fit in better in more punk styled clothing. I've seen girls in bikinis, preps, and a girl dressed like an Angel of Darkness (all black with black feathered wings), so you'll be fine regardless. Just be yourself, and wear good shoes. Your feet will hurt no matter what, tbh. If you're a guy, same guidelines: shorts, good shoes, light colored shirt. A lot of guys come shirtless, which is fine, but you'll get burntttttt to a crisp if you're pale.
Vans because it's Vans warped tour or you could go like Kellin Quinn with Toms any walking shoes are cool, jeans or shorts I wouldn't wear a skirt that's no, especially not in a mosh pit... A band shirt would be fine. These are my opinions go with whatever you feel most comfortable with:)
ripped short shorts and a crop top