If the girl is working, like a waitress, receptionist, salesgirl, etc. several different situations can occur. First, if she is not busy or occupied with someone else, you can use a fairly basic approach, like, "Hi! looks like you have time on your hands! Aren't you usually busy this time of day?"

Then after she replies, you can get right down to business. Say, "You know, when I was walking past, I noticed you, and I decided I’d like to meet you. My name is. ..., what's yours?" If the girl has a co-worker nearby, within earshot, don't worry - just proceed as if they weren't there.

If she is busy, and you can't get to talk to her without other customers hearing, about the best thing you can do is say, "I'd like to see you later. Would that be possible? What would be a good time?" Then the girl won't be too embarrassed, since the customer doesn't know whether you already know her or not. A date can be made then, or later.