I may say "I am fine/everything´s okay." or something like that, what comes first to the head. It does replace "how are you?" which, in turn, seems having replaced "how are you doing?"
Now one experience of mine:
In had just entered a chat room where just English were spoken. I knew far less than I know today, my repertoire was almost nil, then I said "What´s up? "
Indeed it was intended to get a reaction, to know how to answer that question in the future...but then, some funny guy answered me, and the answer was...The sky, the clouds, the planes, the stars...
It took really a while until I understood the sarcasm. When I did, the chatter had left for hours (or it seemed) and then, I realized that it was a joke! and then I was like HA HA HA...
In plain English wazzup means "o que tem/está lá em cima?"
Most of the time they won´t kid you, but I let you know just in case.