谁 = 讠(speech) + 隹 (a short-tailed bird, lending sound). 'Who' is something you ask out loud, so it carries the speech radical — the same one in 说, 谢, 读.
谁 has two accepted readings: shéi (more common in speech) and shuí (more formal/literary) — both are correct, so don't worry which you hear. It asks 'who': 你是谁? (Who are you?), 谁是老师? (Who is the teacher?). It can also mean 'anyone / whoever': 谁都可以 (anyone can).
The speech radical (讠) — a question you ask aloud: WHO?
谁 is read both shéi and shuí — both are 'correct'. Native speakers mostly say shéi in conversation, shuí in formal reading. Pick either; you'll be understood.