ok...I'm not such a good one to give advice either since I really suck at picking up guys (and girls thank god!
) But Everyone has given good pieces of advice so far.
I'd say go in looking for a shirt...and ask her for her help. Grab a couple of shirts, try each one on, asking her which one she thinks looks best on you. Tell her that you are trying to pick out the most perfect shirt becuase you are hoping to look your best for a first date with this girl that you are trying to get to know. She'll tell you which one looks best, and you say "Great! Thanks for all your help....oh by the way....you're the girl that I was hoping to go out on the date with. Would you give me the pleasure of your company?"
I dunno if that would work on all girls, but if a cute guy came up to me and did that I'd be pretty flattered...it would show that he cares about what he looks like. This interaction also gives you the opportunity to get to know her a lil bit and to see if you would like going out with her.