Whom replaces a subject where the answer to the question could be either “him” or “her” or “them.” I remember it like this: whom ends in an M, just like him.
Q: From whom did you take that book?
A: I took it from Jake.
Because the pronoun for Jake is “him,” you use “whom.” (That is, if you couldn’t say Jake’s name, but could only point to him, you’d say, “I took it from him.”) But for everything else, you use “who.”
Q: Who is going to make dinner?
A: Cindy is going to make dinner.
The pronoun for Cindy in that answer is “she,” not “her.” If you just pointed at Cindy as your answer, you’d say, “She is.” So you use “who.”
Bonus knowledge! Good Catholics should already know this rule by heart from the rousing gospel song “Whom Shall I Send?” (Answer: I shall send him. Or Him, if you want to be technical.)
So there you are. You’re welcome.