"What's up?" is a casual greeting that basically means "What's happening in your life right now?" "Not much, how about you?" is indeed the standard answer to this question. But "What's up?" isn't quite the same as "How are you?", so I wouldn't answer with "I'm fine, thank you." "What's up?" can also be used in the context of asking someone what's wrong. You can say it to someone who looks sad or frustrated, for instance.

It's almost the same as "How are you?" in English....a casual informal greeting...and the probable answer by you in the question is the answer to this question..... "Not much , how about you?"...

Most common replies to What's up ? in Ireland would be: Not much, what about you? Ah the usual, and yourself? You know yourself same old same old, how about you?

Thank you a lot. Know I can see but maybe it's kind of ... foolly? but can you give me some examples what should I say?