Nice to meet you

 hi,how are you?

还有how do you do,用于很正式的场合

it isuseful   ! thank you

>>when someone greet to you by saying" what's up?" how do you reply?
Any way you like.

You can tell them what is new, what you are doing, or that you are not doing anything, or why you are there, ...
Sometimes "what's up?" is used to mean "What is the problem?", but also used as a general greeting & invitation to speak.