So I translated it like: "Ni zai nar? Ni zuo shenme?" But I found "Ni zai zuo shenme?" online... How is it zai zuo (in + to do)??? And is it correct if I just say zuo (to do)?? Does it express the right meaning?
We usually don't say "你做什么(ni zuo shen me)". We only say it when we are shocked, like when you just meet a man trying to robber you, you'll say that to him.
If you're asking someone what are you doing, you should use "你在做什么(ni zai zuo shen me)".
There is more than one way to skin a cat and more than one way to express oneself. Here a couple of suggestions which may fit in certain circumstances. You can listen and read the words.
Listen here(张国荣) Where are you? 你在何地
Listen here Where are you all now? 你们如今在何方
What are you doing?!
你在干什么？ What are you doing?
What the hell are you doing? （a little angry) 你到底在干吗？