In Korea, you can say 안녕하세요(Annyeonghaseyo) or 안녕(Annyeong) for "Hi."
In Korea, we use two forms when we are talking.
When you are talking to older people and someone not familiar to you or just you want to respect the person, you use 'High Form.'
When you are talking to your friends and younger people or someone who are very close to you(even though they are older...like your mom, dad, brother, sister, or some older friends), you use 'Low Form.'
Some parents strictly make their children to use 'High Form' to them. It sounds more respectful.
안녕하세요(Annyeonghaseyo) is High Form.
안녕(Annyeong) is Low Form.
And then you might want to say "만나서 반갑습니다(Mannaseo Bangapseumnida)." or "만나서 반가워요(Mannaseo Bangawuoyo)."
It is "Nice to meet you." in Korean.
When you know the person and meet her or him, you can say "How are you doing?"
In Korean, it is "어떻게 지내세요?(Eoteoke jineseyo)"
Then, the answer may be "I am doing fine."
In Korean, it is "잘 지내요(Jal jineyo)." or "잘 지내고 있어요(Jal jinego iseoyo)."