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.

If she is busy, but there are no strangers within earshot, you can ask her name and introduce yourself, and ask her if she is busy later. This is suitable with waitresses, receptionists, office workers and some others. A date can be made then, or later.

In both cases mentioned (with the girl busy), if she should say "No," you should be a little persistent.

You should say, "Oh! Come on now. I only want to talk to you for a minute or two. You can't be that busy, all the time." Her "No" may have been a knee jerk reaction, because you took her by surprise. She will usually relent, unless she has a strong feeling about not wanting to meet you.