Ah ok... I was gonna say a NOFX tshirt until I read Chris' answer. Not sure if they still do the tour, but back in the day they were the act I was most interested in seeing.
Perhaps an old-school punk band's shirt will earn you a bit more street cred...
To be honest though, the real advice you need in terms if Warped Tour fashion is what NOT to wear.
Firstly, you've gotta be comfortable. It's an all-day show, the temperature is going to be hot, and its going to get sweaty. You'll want to wear clothes you can move around easily in, and nothing you feel too precious about either.
Footwear-wise, I guess Vans (the main sponsor of the event incidentally) or a similar skate brand is the way to go.
Forget about sandals or flip flops if you value your toes.
You'll also want to remove any hoop earrings or anything that's likely to catch. Even if you don't intend to mosh, you never know when a mosh pit will find you!
Plus, it's going to be very crowded anyway.
If you're worried about fitting in, I'd recommend checking out some pictures online to see what everyone else rocks up in.
A lot of the girls wear shorts and a bikini top. Skirts or dresses are a bad idea though.
If you're interested in standing out from the crowd, I wouldn't worry too much about the clothes, but rather focus on the amount of ink on your skin or what color UV reactive hairdye you're going to use.