Learn Russian: Где парк? - Where is the park?

Look at its ending! In Russian, you can usually identify a noun's gender by its last letter. The word парк ends in a consonant (к). Nouns that end in a consonant are masculine.

Why is there no word for "is" in this sentence?

In the present tense, Russian almost always leaves out the verb "to be" (am, is, are). So instead of saying "Where is the park?", you simply ask Где парк? (literally: "Where park?").

How would I ask "Where is a park?" instead of "the park"?

It would be exactly the same: Где парк?. Russian doesn't have articles like "a", "an", or "the". You just use the noun, and the listener figures out if you mean "a park" or "the park" from the context.