Jina's answer was great. I'd just like to mention that you should avoid using the word 'you' in Korean, unless you are very intimate with the person and they are younger than you. As an alternative, as Jina mentioned, use the person's title or name+씨.
even with dudes I have known for years, I always say 형, and never 너, and definitely never ever 당신, cause they'd probably think I was hitting on them
If you don't know their title or name, use 언니, 어머니, 아줌마, 아저씨, or even 선생님 (even though they aren't a teacher).