One common mistake I have noticed that most people make while learning english, is that they think 'Whats up' is an alternative for 'how are you'. That is false. It is actually an alternative for 'what are you doing'. 'Whats up is a phrase in English that has been shortened. The entire phrase is 'What are you up to?' So the next time somebody says 'what's up?', they are really asking 'what are you doing?'

"What's up?" means "What's happening (with you)?" Answer it as you would if asked that question. It calls for a brief answer though, when used as a greeting. "Hi. What's up?" "Not much, what's up with you?" "Oh, nothing, just got back from the store."

I think it is situation based question... and many time it doesn't require any answer..Just say "Nothing", "fine" or say what are you doing at that moment like "nothing, just chilling out with friends. How are you??" I am just presenting my view for the question....Please comment anyone if i am wrong somewhere, as i am also learning English.

Please take a look at this: http://english.stackexchange.com/questions/2471/what-is-an-appropriate-response-to-whats-up-greeting On that webpage, most of the people say that "what's up" really is just a greeting, and doesn't require much of an answer. Personally, I think that you could give an answer to it. Your answer would depend on what you have been doing. "What's up?" -- "I was just fired from my job." Or: "Not a lot. I've just been staying at home watching TV." My point is that there is no "standard" response.

What's up?有什么新鲜事？ A:What's up? B:Not much. /Nothing special.