don't know what to doUsed after a question word or verb phrase to say that you don't know what would be best to do. The polite form is したらいいか分かりません. The verb before したら can change depending on the action, as in どう説明すればいいかわからない (I don't know how to explain it).
JA: 明日の発表で何を話したらいいか分からない。
EN: I don't know what I should talk about in tomorrow's presentation.