A common greeting in the US is "What's up?" But if English isn't your native language,
you might think, "I don't know, the ceiling fan?" You know what? When people ask this question
they're not literally asking what's up and they're not even asking how we use it idiomatically
which is to say "What's going on with you?" It's just another way to say hi. They are literally
just saying hi when they say "What's up?" So, how do you respond to
the question "What's up?" The most common easiest answer, the one that's
most expected is "Nothing" or "Not much". What's up with you?
So say nothing or not much and then return the question, that is generally how we respond to this greeting.