Who:
* Who is coming to the party?
* Who wrote this book?
* Who are you going to the movies with?
Whom:
* Whom did you see at the store? (formal)
* Who did you see at the store? (informal)
* To whom should I address this letter? (formal)
What:
* What is your favorite color?
* What did you do today?
* What time is it?
Which:
* Which one do you want?
* Which book did you choose?
* Which way to the park?
Whose:
* Whose car is this?
* Whose dog is barking in the yard?
* Whose idea was it to go to the beach?
Note: "Whom" is often replaced with "who" in informal language, especially in spoken English.