Well, I'm not a native speaker, but I'll give it a go!
Did you mean "뭐하는 거야?" That means "What are you doing?"
Firstly, '거야' is in the informal speech, so you probably shouldn't use this with someone older or of a higher title than you.
Secondly, in this context of "뭐하는 거야?", the 는 거야 is used to indicate present tense.
For example, "뭐하는 거야?"- What are you doing?
"공부하는 거야."- I'm studying.
You can also use 거야 when referring to the future tense. However, you must conjugate the root verb.
For example: 하다- to do.
" 뭐 할 거야?"- What are you going to do?
If there is no consonant ending the word, you use 'ㄹ 거야'
If the word DOES end in a consonant, you use "을 거야." (for example: 먹을 거야)
The formal form of '거야." is "거예요."
For example: "선생님, 오늘 뭐 할 거예요?"- Teacher, what are we going to do?
I hope I helped you! If you're not satisfied with my answer, then message me and I will ask one of my native speaker friends to help you!