Well, since its a concert, maybe a band t-shirt with a nice pair of jeans and some flats? Or, if it'll be cold, you can never go wrong with la<x>yers (A lacy tank-top under a regular shirt). I would wear jeans for sure, because skirts are too dressy for a concert. Just dress in what you would normally wear to go out with girl friends. <br />
Best of luck, and I hope you have a good time :)
Be yourself.....if you want to look cute yet be comfortable, that's how you should dress. Don't over think this.