Learn Turkish: Harfler duvarda. - The letters are on the wall.

The locative suffix (meaning in, on, or at) follows 2-way vowel harmony. Since the last vowel in duvar is 'a', the suffix takes the matching back vowel, becoming -da. If the word had a front vowel (e, i, ö, ü) like ev (house), it would be evde.

Where is the word for 'are' in this sentence?

In simple Turkish sentences describing where something is, you often don't need a word for 'is' or 'are'. The 'are' is simply understood from the context of putting the subject (harfler) and the location (duvarda) together.

Does harfler mean letters you write to someone, or alphabet letters?

It means alphabet letters. The singular word is harf. If you want to talk about a letter you put in an envelope and mail to a friend, the word you need is mektup.

How would I say 'on the walls' if the letters were spread across multiple walls?

You would stack the suffixes, always putting the plural suffix before the location suffix. The word for wall is duvar, the plural makes it duvarlar, and adding the locative 'on' makes it duvarlarda.