How do I start dating this girl?

[Lucas]

Hey, so i've been talking to this girl from my school on facebook for a week, but i've only seen her personally 2 times, cause our schedules don't really combine, i'm interested in her, but i'm not sure if she is interested in me, she called me cute and other stuff. I want to ask her out, how should I do it? should I ask for her number? And what's the best way to ask for it?
Thanks in advance
a coffee shop seems like a good idea, but how do I prevent awkward silences, and how should i ask for her number?