oh...
well, if I may suggest something;
It's okay to not react in a big way-- to be more cool and calm about it (create a mystique) because some girls like it that way, anyways.
Kind of like playing hard to get.
If you can't manage words, you could always use bodylanguage.
If you're too nervous to really move, just try your best to act stoic or aloof rather than petrified.
Eventually, you would need to talk with them normally and perhaps give out your email or phone number - and after THAT, you will need to actively do your best to speak to her... but flirting isn't really an issue if you're the one being flirted with (not 100% of the time, anyway...)-- Think of it as meeting them half way already.
They've done the initial work; you just talk casually and if you like them, ask them if they'd like to hang out?
If you have a sister or a female friend, you should practice what you'll do- I'm sure they can help you, even.
But seriously... getting hit on is a good thing- means you don't have to hit on them to get their attention because you've already got it.