The thing is, our private areas mislead us. We're so taken aback by someone we fancy/find alluring that we forget they're just people too.
Really, the worst that can happen is that they reject you in a rude or cruel manner.. And who TF wants to be with a person like that anyway? That's called dodging a bullet.
If they simply reject you, that's also a blessing in disguise.
As for starting up the conversation, remember they're just a person. How would you like someone to approach you?
Don't go up to them with a million questions in your head. Just one to start, then a compliment is a nice ice breaker (only genuine ones).
Even if you're shy, put on a brave face - confidence, NOT cocky, is very attractive.