The simple answer: Always use 'who'. 'whom' is getting less and less commonly used. But - if you do need to use a more strictly correct, and formal style, then use 'whom' as the object of the sentence, and 'who' as the subject. There are many grammar guides online that you can search that explain it well, and give examples.

Who is used parallel with he or she. Eg: Who is going out this evening? She is going out. Whom is used in connection with him or her. Eg: To whom does this t-shirt belong? It belongs to her. cheers