I can guess it probably means "what's happening", or "what's up". Am I right? I am curious where this phrase comes from and how to use it properly.
The phrase is Afro-American in origin, from the 1950s. From 1970's Black Slang: a Dictionary of Afro-American talk By Clarence Major:
Here's some published uses from the 1950s.
From Sig Byrd's Houston by Sigman Byrd, 1955:
There's also this from 1931, it could be related but it's probably not:
Anyway, as to how to use What's shaking?, the excellently titled Flappers 2 Rappers: American Youth Slang (1996) gives some variations: