There isn't really a direct translation of "what's up", but you would say "Wie geht's?",which means "How are you". And as a reply you could say "Danke, gut" (Thanks, I am good), or if you want it to be a bit more slang/colloquial you can say "Alles klar" (Everything o.k.).

bitte schön :)

Danke :)

Wie geht´s? Mir geht es gut.