Learn German: Setz dich bitte!

In German, you do not just sit down, you 'seat' yourself. The verb setzen literally means 'to set' or 'to place'. Because you are doing the action to yourself, it needs a reflexive pronoun. dich is the accusative reflexive pronoun for 'yourself' when talking to one person informally.

I thought the word for 'to sit' was sitzen. Why are we using setzen?

This is a very common mix-up! The verb sitzen describes the state of already being seated. However, sich setzen is the physical action of sitting down. Since you are telling someone to change positions and take a seat, you must use the action verb setzen along with its reflexive pronoun.

Why is the verb just Setz and not setzen or setzt?

This is the informal command form (the imperative) used for du. To tell one friend or family member to do something, you take the verb stem. The stem of setzen is setz, so that is what you use here to give the instruction.

How would I say this to a stranger or a group of friends?