Here's a helpful tip I always told my English students: use "whom" where you could use "him". Use "who" where you'd use "he" .
Examples:
1. To whom should I write this $5,000 check? To him? Really, I should mail this check to him??? You must be out of your mind! I'd rather give it to whomever won't waste it on donuts!
2. Who ate the last donut? He ate the last donut?