How do I decide between who and whom?

In spoken English and in some informal writing, whom is rarely ever used, but in formal writing, you must determine whether to use who (nominative) or whom (objective) when introducing a relative clause. To help you recognize whether the pronoun is being used as a subject (nominative) or an object (objective) within the relative clause, try rephrasing the clause as a sentence and substituting a different pronoun for who/whom.
I met the artist [who/whom painted this picture].
Rephrased: She painted this picture.
Correct: I met the artist [who painted this picture].

In the above relative clause, she is the subject of the verb painted; therefore the nominative case is required.

He is the kind of person [who/whom I admire].
Rephrased: I admire him.
Correct: He is the kind of person [whom I admire].

In this second example, him is the object of the verb admire; therefore the objective case is required.

Remember that the key question is how the pronoun functions with the clause.