How Do I Ask Out A Cashier?

Ok so there has been this really cute cashier girl working at the Target by my place. She's been working there for several months but only recently have I found myself flustered by her. I talked to her once last week but it was only for 2 minutes because there is a line of people behind me. I talked to her again today and I was surprised to see that she remembered me, but this time we only talked for less than a minute because I got way nervous. I want to ask her out but I don't if it's too soon (I mean we haven't even exchanged names, I only know hers because of her name tag) and I'm worried that the constant presence of people will make her feel pressured and therefore shoot me down. So should I ask her out next time I see her? Should I get her number first? Should I wait after a few more meeting? any opinion would be appreciated.