Always be confident and don't put her into a stressful situation or she'll instantly try to avoid confrontation with a no or silence.
At this age the girl (if she's the same/similar age as you are) probably isn't too experienced with guys being confident enough to ask her out directly in a store like that. She'll be impressed you even had the guts to straight up ask her, unless she really gets around and is full of herself, at which point she not good enough for you.
Now, I would suggest not directly asking for her number off the bat. Engage her and make good eye contact and ask a question about an item. use small talk to see if she's even remotely interested. If she's just like "another customer" and doesn't even make eye contact with you, she won't be comfortable with someone asking her out at work, or maybe she just isn't interested (which isn't a problem). If she is eager to answer questions and engages you, you can move on to some small talk. This doesn't mean a conversation, it can be a simple question or something about yourself that connects her with you and makes you stand out as more than just a customer.
Then you can ask her out. I would suggest asking if she would want to do something, and then get her number as a result (as contact information) rather than just being creepy and asking her to give out her number to a stranger.
Go with something casual, as people have said. Coffee or lunch, anything where the setting isn't too intimate and it appears to be a more casual hey I want to get to know you, not hey I want to get in your pants. It also allows for security. She knows if she;s in a public place there's less to worry about.
How you ask the question is a little more complex. Keep it simple, and keep it casual. Avoid yes or no questions. After you've established a connection socially, say something like "We should hang out sometime". This does not directly ask her "Do you want to..." it implies that you're a sociable character and it's normal for you to hang out with people, and also doesn't force her to say yes or no.
Always be confident. Don't bring up the option of rejection. If she does reject you, keep your spirits up and if it wasn't too harsh perhaps pursue just a friendly relationship, as it's easier for her to accept an invitation when she knows you as friend.
Ultimately, I think you should receive her number as a byproduct of the interaction, not as the goal of it. Thus the woman feels appreciated and connected to you rather than just another chick in your contacts.
Confidence and Casuality(?) are key. Be chill, it shouldn't be a big deal. The less pressure on her means the more appealing you come across, and the easier it is to say yes to you, rather than scaring her off with a really blunt or creepy request.
My 2 cents (or maybe dollars, this things a book eh?).