Wo-compounds are used to pose questions.
As with da-compounds, they may only be used if the object of the preposition is not a person.
The preposition is preceded by a wo (-r-).  Use the 'r' if the preposition begins with a vowel.

                 Worauf wartest du?  --     Ich warte auf den Bus.
 

As with the da-compounds, if the object of the preposition is a person, you must use a pronoun.
Here, the pronoun will be an interrogative pronoun.  The case depends upon the preposition.
 
 Nom       wer          Wer ist das in der Ecke?  [Who is that in the corner?]
 Acc        wen          Wen kennst du auf der Party?  [Whom do you know at the party?]
 Dat         wem         Mit wem ist er gekommen?  [With whom did he come?]
 

The preposition precedes the interrogative pronoun.
As always with questions, the interrogative (or question word) begins the sentence.
 
 

 Modell:  Ingrid wartet auf den Bus.  Worauf wartet Ingrid?
                                    Akk  Object  ==> wo-compound
 

   Ingrid wartet auf ihren Vater.     Auf wen wartet Ingrid?
                           Akk   Person   ==> prep + interrogative pronoun