Who and Whom
Who is the subject of a verb. A simple way to check whether who is correct is to replace it with a subject pronoun (he, she, they).
Kate is the girl who won the competition.
We can say 'she won the competition', so who is correct.
I have not met the man who lives next door.
We can say 'he lives next door', so who is correct.
Whom is an object. You can use whom if it can be replaced with an object pronoun (him, her, them).
This is the man whom I met yesterday.
We can say 'I met him yesterday', so whom is correct.
Learn more about pronouns.
Knowing when to use who and whom is difficult for many native English speakers, and whom is becoming less common. If you are not sure, it is safer to use who.