Learn Russian: Это твоё письмо? - Is this your informal letter?

Possessive pronouns like "your" have to match the gender of the noun they describe. The word for letter, письмо, ends in -о, which means it is a neuter noun. So, we must use the neuter form for "your": твоё. For comparison, твой is for masculine words, and твоя is for feminine words.

Is это also in the neuter form here to match письмо?

Actually, no! In this sentence, это is acting as a set phrase meaning "this is" or "is this". When used this way to introduce or point to something, это never changes its ending, no matter what gender the following noun is.

Why isn't there a word for "is" in this sentence?

In the present tense, Russian almost always skips the verb "to be" (am, is, are). The sentence literally translates to just "This your letter?".

How do you make this a question when speaking, since the word order is exactly the same as a statement?

You just use your voice! To ask a yes/no question in Russian, you keep the exact same word order as a normal statement, but your voice goes up in pitch sharply on the specific word you are asking about (for example, on твоё if you are asking if it is your letter).