A standard response to 'what's up' would be 'not much' or 'nothing much'. If you want to give a sarcastic and silly response you can say, 'the sky'. :P

What's up? = What's happening? You answer it like any other "what" questions. Nothing; nothing much; not a lot; (all very casual) If something great has happened then you can tell your friend about it.

(^ w ^) When someone says "Whats up" I personally would say its kind of like slang. Its basically informal, people would generally say this kind of greeting if there acquainted with the person. As far as answering greeting, you can say nothing much, or your current status Does that make sense? lol

"What's up?" is like saying"what are you doing?" or "What is happening?" So you could respond with "not much" or "browsing italki"......something like that

you can tell him/her"what's going on" here, but if nothing serious, you can just say nothing/nothing serious~