Ok, here's the simple trick to help you remember. If you can answer the question being asked with him, her, them, me, or us, use whom. Just remember whom and him both end in m. For example, Who/Whom were you talking about? You were talking about him, so you would use whom.
The same trick is applied to who. If you can answer the question being asked with he, she, they, I or we, use who. For example, Who/Whom is arranging the conference? She is arranging the conference, so you would use who.
As always, if you have any questions or I can help clear anything up for you, leave a comment below!