Someone will tell you "whom" is dead. In speech they are very nearly right — nobody says "whom did you call" outside a costume drama. In writing it is alive, and using it wrongly is worse than not using it at all. A misplaced "whom" reads as someone reaching for a formality they have not quite got hold of, which is a more conspicuous error than simply writing "who" throughout.
The subject form. Use it when the word is doing the action.
The object form. Use it when the word is receiving the action, or after a preposition.
Quick way to remember
Answer the question with "he" or "him." If "he" fits, use "who" — both end in a vowel sound. If "him" fits, use "whom" — both end in "m." "Who called?" — "He called." "You called whom?" — "You called him." It works every time, including in the middle of a sentence.