Someone
noun
誰 is who, 誰か is like asking "who?" If you don't know who it is, you just know it's someone.
だれか
The reading comes from the reading you learned with the kanji.
「あ、あそこに誰かいる。」「え、誰もいないよ？」
"Oh, there's someone over there." "Uh, there’s no one there…?"
誰かに聞いてみよう！
Let's ask someone!
チームの誰かが代表として相手とじゃんけんします。
Someone on the team will play rock Rock-Paper-Scissors with the other party as a representative.