내가 알아야 할 게 뭔지 말해.

English Translation

Tell me what I need to know.

게 (ge) - This is a particle used to indicate an action or task. It is attached to the verb stem to form a noun phrase that represents something to be done or an action to take.

“할 게” can be translated as “something to do” or “something to be done.”

example phrases using the verbs with “게”:

여행을 가볼 게요. (I will try going on a trip.)
사진을 보여줄 게 있어요. (I have something to show you a picture of.)
잃어버린 물건을 찾아볼 게요. (I will try looking for the lost item.)
새로운 언어를 공부할 게요. (I will study a new language.)
그 문제를 이해할 게요. (I will understand that problem.)
프레젠테이션을 준비할 게요. (I will prepare the presentation.)
조금 생각해볼 게요. (I will consider it for a moment.)
English Translation

Tell me what I need to know.

뭔지 (mwonji) - “뭔” is a contraction of “무엇인” which means “what.” “지” is a particle used to indicate a question. Together, “뭔지” means “what it is.”