Come from the urban term "You dont want it" which is HOOD for "You're not ready for ME".
The proper English equivalents of "Who Want What?" would be "Who's Next?", "Who Can Fuck With Me?", or "None of y'all can fuck with me!"
"Who Want What" was also the title of a song by Memphis Bleek and Jay-Z where it was used in the same manner on the chorus.
The term is also used in Asher Roth's song "College". He mentions walking in a party and seeing people playing pool, to which he exclaims "who want what!"
(Two kids playing video game against each other)
kid1: yo ass think you could fok wid me? watch this... bang bang!
kid2: bang my ass, you already ded mafuka! WHO WANT WHAT!!
(Guy1 wins just won basketball game in playground)
Guy1: These dudes was no competition!!
(looks at the next batch of players coming to play against his team)
Guy1: Yall seen me? I just scored 70 on Guy2's team! Dont act like yall aint see! Which one of yall want WHAT? Who want what? Who the FUCK want WHAT!