If in doubt, use "who", as Su.Ki. says. You can't really go wrong with it in informal usage. But the reverse is not true --- you really don't want to overdo "whom" and use it when "who" is called for. It's especially easy fall into this trap with relative clauses and where the pronoun looks vaguely like an object. You may see things like "The police are interviewing a suspect, whom it is believed was in the vicinity last night" or "Whom shall I say is calling?", which are incorrect, even if you do like to use "whom" when possible.

A continuing debate in English usage is the question of when to use who and when to use whom. https://www.italki.com/discussion/110190

The word who can only be used when it is the subject of a verb. That might sound confusing, but it just means it is like the words I, he, she, we, and they. Just like who, each of these words can only be the subject of a verb. The difference with who is that some people are unsure when to use who and whom. Well, confusing that pair is no different from confusing these pairs: I/me, he/him, she/her, and they/them. Example: I have not seen the man who lives in the hut by the beach for a week. (Who is the subject of the verb to live.) Whom is never the subject of a verb. (Who, on the other hand, is always the subject of a verb.) Example: You sat by whom all night? (In this example, whom is not the subject of any verb. You is the subject of to sit.) Always use whom after prepositions. (Prepositions are words like to, with, by, on, in, near.) Examples: You have a child by whom? (by – preposition) With whom did you see Janice? (with – preposition)

'Who' refers to the subject of the sentence - like 'I' or 'we' or 'he' or 'they'. 'Whom' refers the object of the sentence - like 'me' or 'us' or 'him' or 'them'. In everyday speech, 'whom' is almost never used. So while 'Whom did you see?' is technically correct, you will very rarely hear this. Most native speakers use the more natural-sounding 'Who did you see?'. Likewise, you are far more likely to hear 'Who did you speak to?' as opposed to 'To whom did you speak?'. The form with 'whom' is, strictly speaking, the correct form, but it sounds stilted and unnatural to most native speakers. Learners tend to worry about 'whom' because it's in their grammar books and they feel they ought to know how to use it. These days, the main use of 'whom' is in the middle of a formal sentence following a preposition, for example, 'The police interviewed the man with whom the victim had spent his last hours'. But the simple truth is that you will probably never need to use 'whom'. Unless your job is to write formal documents, you can always manage without using this word.