Why don't you just say hi?

If you need an excuse, buy something so you can talk to her.

You can ask her how long she's been working there. Is she in school or anything now?

Hopefully by now she's said something that you can talk to her about or ask her about.

If she just gives you one word answers and looks distracted then just smile and leave.

If she seems interested and the conversation feels comfortable tell her you know she's busy right now but you'd like to keep talking sometime. Then ask her if you can get her number.

She'll either say 'I don't know' because she has a bf or she's notinterested or 'Haha ok sure'

Either way it breaks the ice and lets her know you like her. She's going to laugh about it anyway after you leave, unless you really are a creep.

Don't go into the situation like you're trying to get a date out of it. Act like you're talking to your sister or cousin. Be friendly and you'll get a friend. Then get to know her a little bit better then ask her on a date.