How do I pop up to her?

I met a girl I like the other week at a party. I was very drunk and can't remember much of the night. When I walked past her in town the other day, she smiled and I just about cracked a smile. I've been thinking about her for a while since then. I'm absolutely useless at social interactions, both in person and online.

I want to pop up to her but I can't bring myself to do it, I'm really paranoid about what to say so for the past couple of hours I've been struggling and still haven't said anything.

I'm 18, and I feel like a bit inadequate not being able to just pop up. My previous experiences are limited, and ended up to friendship at most.

First off, how do I start off the message? Secondly, how do I continue as not to fall under just friendship?