Formal:
* O que está a fazer? (This is the most formal way to ask)
* O que você está fazendo? (Slightly less formal, but still appropriate for most situations)
Informal:
* O que você tá fazendo? (This is the most common way to ask in informal situations)
* O que está fazendo? (Less common, but still understandable in informal settings)
Other options:
* Que é que estás a fazer? (This is a more colloquial way to ask, similar to "What's up?")
* Está ocupado(a)? (This literally means "Are you busy?", but can be used as a casual way of asking what someone is doing)
The best way to choose the right translation depends on the situation and the level of formality you want to convey.