Learn German: Wer schaut aus dem Fenster? - Who is looking out of the window?

The preposition aus (out of) always forces the following noun into the dative case. Because Fenster is a neuter word (das Fenster), the word for "the" changes from das to dem.

Why does the verb schauen (to look) end in a -t here?

When you use the question word Wer (Who), the verb takes the third-person singular form (the "he/she/it" form). Just like in English we say "Who is looking?" instead of "Who are looking?", German uses Wer schaut?.

Doesn't the word aus usually mean "from"?

It can mean both! You have probably seen aus used for origins, like Ich komme aus Deutschland (I come from Germany). However, when talking about looking or moving from the inside to the outside of a physical space, like a window or a building, it means "out of".

Is Fenster singular or plural here?

It is singular. The plural of das Fenster happens to be exactly the same: die Fenster. We can be sure this sentence means "out of the window" (singular) because it uses dem. If it were plural, the dative article would change it to aus den Fenstern.