Learn French: Où sont les toilettes ?

In French, the word for "restroom" is almost always plural: les toilettes. Because the noun is plural, you must use the "they" form of the verb, which is sont (are). You are literally asking "Where are the toilets?"

Since the current lesson is about "à" and its contractions, how would I say "to the restroom"?

Great question! Since toilettes is plural, you must combine the preposition à (to/at) with the plural article les (the). They contract to form aux. So, "I am going to the restroom" translates to Je vais aux toilettes.

What is the little mark over the "u" in "Où"?

That is a grave accent. It does not change how the word is pronounced, but it does change the meaning. With the accent, où means "where". Without it, ou means "or". The accent helps you tell them apart in writing.

Do I pronounce the "s" at the end of "les" or "toilettes"?

No, the "s" at the end of both words is completely silent. In French, you usually do not pronounce a final "s" unless the next word starts with a vowel sound (a liaison). Since toilettes starts with a consonant (t), the "s" in les stays silent.