Normally I'd advise against getting dressed up and wearing heels, but it'll most likely be in an amphitheater or stadium or whatever, so you could probably get away with it (if you're used to standing in heels for a long-ish period of time, anyway) but I imagine that you might be a little overdressed if you do. Just wear your normal clothes, haha.
uhh well I don't really listen to her so I don't know what her concerts are like-but rock and rap concerts(only kind I've been to)-you CANNOT wear heels,unless you want to break your legs and sh*t...but I'm guessing pop concerts are a bit more calm,try looking up her concerts on youtube and look at what the crowd is wearing
I've only ever been to small concerts in local clubs, but I definitely wouldn't suggest wearing heels unless you're ready to stand in them all evening. I think you'd be alright if you just wore jeans, comfortable shoes, and a cute top.