"Who are you going with?"
"Which box did you put it in?"
"Who's the letter addressed to?"
Do the above sound perfectly correct to you? Well, they're not. They're certainly accepted in everyday speech, and it's fine if your characters talk that way. But each of the above is technically incorrect, because each one ends with a preposition devoid of its object. In short, it "dangles."
(Side note: If you were never forced to memorize your prepositions, DO IT NOW. I had to memorize them in seventh grade, and I've never forgotten them.)
When using prepositions, they must always -- ALWAYS -- be followed by their objects. They may not stand alone. (Remember, too, that certain prepositions can be used as other parts of speech, so it's important to understand HOW they're being used before assuming they're prepositions.)
Here is the CORRECT way to write the three examples above:
"With whom are you going?" (Preposition WITH followed by its object WHOM)
"In which box did you put it?" (Preposition IN followed by its object BOX [which = modifier])
"To whom is the letter addressed?" (Preposition TO followed by its object WHOM)
My above examples are all interrogative (questions), which doesn't need to be the case for this rule to apply:
"I'll let you know for whom to ask at the front door." (Preposition FOR followed by its object WHOM)
And as an added bonus, here are the prepositions for you to memorize. *grin*