How do I ask this coworker for her phone number?

So I am 23 years old and the Assistant Manager in this shoe store, I am the only guy in a store full of girls mostly between 18-20 years old. There is this one girl in particular who i think is pretty cute, not just her looks but her personality, I joke around with her alot and to be honest I know it sounds lame at my age but i have a huge crush on her.

She is 20 years old, she is pretty inexperienced wise, because her family is pretty religious, she has only has one boyfriend which she got at 19 which she recently broke up with and i think she is still getting over him, and she is from a small town in the outskirts of town. She doesn't really know anybody in town, and she is here for school so everything is pretty new to her. So i guess my question is how do I approach this, I need some ideas and even tips. I know I am her boss but I don't plan on staying long and i am tired of missing ou on chances with women.