Use who in the place of the subject of a clause.
He is the one who told me about the party.
Use whom in the place of the object of a clause.
To whom should I address this letter?
Tip:
Use he/him (or she/her, etc) as a test:
If you would use the word he in the clause, use the word who.
He told me about the party.
If you would use the word him in the clause, use the word whom.
Should I address this letter to him?