You can wear a skinny jeans with sleeve less tops and boots.
I would wear shorts and a tank top because you'll be in a crowd and it doesn't matter if its outside or inside because you have to remember other people's body heat plus if you're moving around; its gonna get really hot.
If it's going to be warm, I'd recommend shorts or a dress of some description. If you're going to be jumping and dancing, you're going to get hot and bothered and you don't really want anything clinging to your legs to make you hotter. For the same reason, I wouldn't suggest going in your best outfit. I made that mistake for my first concert and it pretty much got ruined by having drinks spilled over it and being sweaty and all the rest. I'd say just a baggy top and some shorts or a nice summer dress. Shoe wise, you appear little, so if you want to wear heels that would look cool. However, heels would probably hurt your feet after a couple of hours and when you're in there, nobody really pays much attention to your attire, especially shoes. I would just opt for something comfortable, like Converse or sneakers or sandals. Have fun at the concert.
What concert? I usually just wear T- shirt and jeggings but i'm not very stylish if you want to fit in with the other pre teeny girls try a damn crop top with high waisted shorts. ((i hate that style))
I would wear a nice pair of jeans with a nice top and high heels.
