To work out whether you should be using ‘who’ or whom’, try this simple rule and see what seems to makes sense.
Notice the two Ms – it helps you to remember the pairs: WHO/HE and WHOM/HIM.
If you feel uncomfortable with ‘whom’, it’s usually possible to rephrase the sentence to avoid using this perhaps old-fashioned term.
Don’t write this though:
Who should I send the bill to?
Otherwise, you are ending the sentence with a proposition! That will never do!
Instead, add more information:
Should I send the bill to you?