Personally, I wouldn't bother with using "What up?"
...without the 's.
Most people consider it ignorant sounding and very specialized slang, used by rappers and among unschooled gang members, if you catch my drift...
On the other hand, "What's up?"
and "What's happening?"
are both very well established and common in A.E. very casual speech as greetings between friends and which don't necessarily demand an answer.
Probably the most famous line spoken by Bugs Bunny*
in his inimitable Flatbush (Brooklyn) accent is:"Eh... What's up, doc?"
...usually said while nonchalantly chomping on a carrot
*Bugs Bunny is a beloved American animated cartoon character invented in 1938 and owned for many years by Warner Bros Cartoons. His distinctive vocal styling was created by performer Mel Blanc.