Well i go to a fair amount of concerts and i see people wearing all sorts of styles...so i think it's really up to you! If you're going to be doing a lot of standing as people typically do make sure yours shoes are comfy! And make sure whatever you wear it's light and breathable since all that body heat around you gets hot!
But other than that i agree with Dani, we need more info! Let us know more about you, what's in your closet etc.