How to approach a certain girl

There's a girl I really like but I'm not sure how to approach her. The thing is she works in a bank and there are 5 tellers in the department she works in so I'm worried that she might be negative if I speak to her as her colleagues are right next to her. The thing is I don't even have an account with that bank, I don't know the girl at all. I'm not scared of talking to her, it's just that I'm not sure how to approach her and besides how would we have a private conversation in a bank? I don't know where else I could possibly see her. I'd appreciate any advice. Thanks in advance.