I would first ask her if I could join her . It's important because you're invading into her personal space. It's important to ask her because lot of girls get offended by it most of times( this is how things are in Europe) Once she accepts introduce yourself and ask things about her , joke with her , use the objects in your environment to make things flow . Grab her phone number to hang out some other time . Say that you're really busy and that you have to go but ask her for her phone number.
If she does not accept any of your proposals, retreat politely and move on. And don't worry about any potential rejection, you get valuable experience out of it.