Greeting

[Dinhtuong]

At the beginning of conversation, people often say:
- How are you doing?
- how's it going?
- what's going on?
- what's up?
- what are you doing now?
- Are you doing Ok?
Could you tell me the usage and meaning of these questions?
What question we often say in formal situation and in formal situation?

[Anglika]

They are "general pleasantries", with no specific meaning [as a rule], but meant to indicate a positive approach to the other person while not expecting a direct reply to the question.

Formally you would say "How do you do?"; the examples you have given are all informal.