hmm, guess you've met a nice Indonesian friend who teach you speak un-formal Indonesian huh? :) that's great.
*lagi apa? or
*lagi ngapain?
that is Indonesian un-formal question, usually it used to friends or someone that close to you.
the formal one is :
*kamu sedang apa? or
*apa yang sedang kamu lakukan?
I guess that's the proper one :)
keep practice, terus berlatih
good luck Oppa ;)