I had a girl I wanted to ask out a few days ago. She sits next to me and class, and while texting and FB were an option, in the end I felt it was best to do it in person. It's kind of like jumping off a diving board really high up. You have to stop thinking and just do it. I ended up just asking "Do you want to go out with me sometime, on a date?". I figured if she was interested, she'd say yes, if not, no. Simple and quick, the way I like it.
However, we'd hung out a few times before then, so this situation might be different than yours. And since you're doing in via text, you don't have to worry about being nervous, a huge plus.