I can't understand why I'm responding to one of these threads.

She has a boyfriend, so she's straight. This means that for some reason she actually wants to try to be your friend, so be a damn friend and stop thinking about trying to put moves on her or anything. Go. Shop. Have fun. Get a fucking clue.

Wear a t-shirt and jeans. It's pretty basic attire for any sort of situation.

And if she ditches you, then she's a bitch. Either that or she came to her senses and realized there's no point in talking to you.