When a question word is the object of a preposition, the preposition usually comes at the end of the clause, especially in an informal style.

What are you looking for? (More natural than ‘For what are looking?’)
Who is this present for? (For whom is this present? is extremely formal.)
Who were you speaking to? (NOT To whom were you speaking?)
Which pictures are you talking about?
What are you laughing at?
Who put the cat in?
Who turned the lights off?

Prepositions come at the end of clauses in indirect wh-questions and what-clauses which are not questions.

Tell me what you are looking for. (NOT Tell me for what you are looking.)
What a lot of trouble I have gotten into! (NOT Into what a lot of trouble I have gotten.)

Some questions consist of simply a question word and preposition.

What with?
Who for?
What about?

Note that this structure is unusual when there is a noun with the question word.

With what money? (NOT What money with?)

Exercise

Fill in the blanks with appropriate prepositions.

1. What are you crying ————–?
2. Who shall I give this ——————?
3. What are you waiting ——————?
4. Which writer were you talking ——————?
5. Which candidate have you voted ——————-?

Answers

1. for
2. to
3. for
4. about
5. for