Whats up means like How are you? or what are you doing? You can reply by saying: Nothing much, followed by action that you are doing. ex: Whats up?! Nothing much just watching tv. how about you? or Nothing really, how about you?

Conversational guides suggest offering a description of what you are/were doing when they ask, or offer a general outline of what is your plan for the day/afternoon/evening, or how is your life/week/month going in general, as to facilitate the conversation (*if* you wish to do it, sometimes it can be considered a lazy move to just message someone and ask 'What's up', so it's also okay to offer nothing and reply with 'Not much' like sakurataisen suggests, and let the other person do some more work).

when someone greeting you by saying what's up just reply by saying not much, because what's up means how are you.