Which is an interrogative, which is a fancy word for ‘question word’. You use it when you are trying to determine the difference between two alternatives (Which was better, the book or the movie?), or the best among a group of choices (Which is the best route to Apache Junction?)
Witch is a noun, commonly used with a negative connotation. It can mean someone who practices either good or bad magic (The Wicked Witch of the West or Glinda the Good Witch in Oz), someone with a bad attitude (What a witch! A phrase commonly used when we really want to use another word that rhymes, but consider ourselves too genteel to utter in polite company). People often dress up as wicked witches for Halloween. The word almost always refers to a female, as a male practitioner of magic is usually referred to as a mage, a wizard, or a magician. The word is also often used to describe an adherent of Wicca.
Keep them straight this way: Witch has a t in the middle of it, just like that other word we don’t like to use. Both witch and the other word always refer to a person. Which has an h in it, just like other question words such as Who, What, Where, and When.