Here's the way you say “Hello.”
안녕하세요?
(The other person will probably say the same thing back to you.) By the way, the word 안녕 means “peacefulness” or “well-being”; this phrase means generally, “Are things peaceful for you?”
The next thing to ask, obviously, is “How are you?”
어떻게 지내세요?
With its response of “Fine:”
잘지내요.