Who and whom is often mixed up.
Who is a subjective pronoun, whereas whom is objective. Who is similar to he, she, it, and they. In other words, using who or whom is dependent on if you're referring to the object or subject of a sentence. Who is used as a subject to a verb, whereas whom acts as an object within a sentence. The same is the case for whoever and whomever.
An easy way to determine this is to use the he/him substitution method. Substitute who with he/she, and whom with him/her.
Who [he] wrote this story?
I called a literary agent, whom I will meet next week. [I called a literary agent, I will meet with him next week]
[this resource should be expanded]