Learn Russian: Что это? Это дерево.

Что means "what" and is used to ask about inanimate objects, like дерево (a tree). If we were asking about a person or an animal (like "Who is this?"), we would use кто instead.

How do I pronounce Что? It looks like it should start with a "ch" sound.

Even though it is spelled with the letter ч (which usually makes a "ch" sound), что is a very common exception. It is always pronounced like "shto" (ш-т-о).

Where is the word for "is" in this sentence?

In the present tense, Russian almost always skips the verb "to be" (am, is, are). So, Что это? is literally "What this?", and Это дерево is literally "This tree."

Does это change to match the gender of дерево?

Дерево is a neuter noun (because it ends in -о), but in this specific type of sentence, gender doesn't actually dictate the form of "this". When you use это to mean "this is" or "it is" to identify something, it always stays as это, even if the noun that follows is masculine or feminine.