Learn German: Du musst dich kämmen! - You have to comb your hair!

dich is a reflexive pronoun meaning "yourself" in the accusative case. In German, the verb kämmen (to comb) is reflexive when you are doing the action to yourself. So, instead of saying "comb your hair", Germans literally say "comb yourself"!

The English translation says "your hair", so why isn't the German word for hair in the sentence?

In English, we specify what we are combing (our hair). In German, the reflexive action of sich kämmen (to comb oneself) already implies you are combing your hair. You don't need to add the word for hair for the meaning to be perfectly clear.

Why is the verb kämmen at the very end?

This is a rule you might remember from learning modal verbs. The modal verb musst (must / have to) takes the second position in the sentence, which forces the main action verb (kämmen) to go to the very end in its infinitive (dictionary) form.

If I want to talk about myself, do I just change du to ich?

Almost! You also have to change the reflexive pronoun so that it matches the subject. To say "I have to comb my hair", you would say Ich muss mich kämmen. The pronoun dich (yourself) changes to mich (myself).