"What's up" is a way of saying hello, although it does mean "what are you doing?" You don't usually answer this question with anything more serious than "hello", "hi" or "what's up" again.

我很好! ， 好!~ 真棒。 基本上，它是 "你好吗？“ = what's up? i'm good, great, awesome.. , not bad.. etc.

The most common responses are "not much" or "not a whole lot"

'What's up' is a way to say 'Hello' to your friends. To answer this, you can say 'Hi', 'Hello', 'Not much' or even 'Hey'.

If you want to improve your English, you should start by avoiding slang like this. Learn the basic foundation of the language first; then you can worry about slang.