1. Make a nice comment on her blouse
2. Take her out to dinner at someplace very nice (probably the combo platter at Sizzler)
3. Finger pick a love song on the guitar
4. Take her to your dojo and pork
5. Wail on guitar...HARD!
In all seriousness, I wouldn't normally just walk up to a random person and say "hi". You have to have something to get "in". Like, "sorry to bother you, I just wanted to say that you have very gorgeous...eyes, hair, smile, etc."
Also, I would NOT start right away with "Hey, do you have a boyfriend". It puts her on the defensive right away...and also she can use the "sorry I have a boyfriend" line if she doesn't want to go out with you.
If you do get a decent convo going, don't blow it and go on too long. Just simple ask if you can have her number, if she wants your number if it makes her more comfortable, or if she'd like to meet at such and such place and such and such time for coffee (don't just right into dinner).