"What's up"/"What's going on", is a quite casual phrase, so you would respond casually as well. Examples being, "Nothing much." or "I'm good". It is also a little bit impolite to not ask that person in return, so those types of responses can followed by phrases like, "How about you?"/"How 'bout you?".
#1 A: Hey Emily, how are you? B: Good, how are you? A: May I come in for a moment? B: Yeah come in.
#2 A: Hey how you doing Chapanya. B: Hey better than yesterday. I'm off my meds. A: Oh wtf you're on meds now? B: I was on painkillers. A: Wtf, it was that bad? Please don't die on me dude.
You have two ways of responding to it: - You can respond with a short answer, another greeting, or not respond at all (just like in #1). - But if you're really close with the person, you can tell em about what's actually going on with your life (just like in #2).