To my mind, this can be the most infuriating word to try to say in Korean.
Depending on the context, it can be
너 "you [a little impolite]"
당신 "you [often overly polite]"
형(used by men) 오빠(used by women) "(older) bro."
누나(used by men) 언니(used by women) "(older) sis."
선생님 "teacher" or "Mr./Mrs."
Most foreigners seem at a loss as to when to use each of these, and misusing them can so easily cause offence in this hierarchy-happy land.
So I've been given this simple advice: grab the person's attention first:
그쪽,...("You there,...") or 저기요,...("Excuse me there...")
Or ask a question of the group or the world in general and wait for the surprised 저요?("Who, me?") that comes up so much in Korean conversations because they have little idea how to say 'you' either...
Then you're free to ask away without ever having to use 'you' again! Brilliant, eh? And after a while hopefully you'll find out their name and what they're happy being called. (Hopefully something more original than 'Mr. Kim'! :p)