When in doubt, an easy way to remember is:
When you could replace it with he, she, or they, use who.
When you could replace it with him, her, or them, use whom.
Who should be should be used to refer to the subject of the sentence.
Whom should be used to refer to the object of a verb or preposition.
Examples of Who:
Who would like to go on vacation?
Who made these awesome enchiladas?
Examples of Whom:
To whom was the letter addressed?
Whom do you believe?
I don't know with whom I will go to the prom.
