CALL her ASAP. Strike while the iron is hot. Asking a girl out is always better on the phone.
As far as what to do if you don't like her:
Just don't hit on her. Don't do anything sexual or flirt with her. And then end the date earlier than you normally would have. Simple as that. Don't overcomplicate things.
I think you'll be fine. You seem to have a good idea of what you need to do. Just do it.
Quote:
...and maybe invite her back to my place. I'll show off by playing guitar,
Don't MAYBE invite her back. If you're feeling her, DEFINITELY invite her back. ALWAYS go for the pull. ALWAYS.
And the playing guitar thing can be GREAT, as long as it doesn't SEEM like your trying to show off.