Learn Russian: Это полиция.

In sentences that define what something is, like "This is [something]", Russian uses the Nominative case. Because there is no action being done to the police, it is not a direct object. Therefore, полиция stays in its dictionary form.

Is полиция considered an animate noun since it is made up of people?

No, words for institutions or groups like полиция (police) or семья (family) are grammatically inanimate in Russian. However, for feminine nouns ending in -я, the Accusative ending always changes to -ю anyway, regardless of animacy. But remember, in this specific sentence, it remains in the Nominative case!

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

Russian does not have articles like "a" or "the", so полиция covers "the police" all on its own. Additionally, in the present tense, the verb "to be" is skipped entirely. You just put Это (This) directly next to the noun.

Since полиция is feminine, shouldn't Это change to the feminine form Эта?

Not in this type of sentence! When used to mean "This is...", это acts as a fixed word and never changes its ending, no matter what gender comes after it. You would only use the feminine эта if you were using it as a pointing word right next to the noun to mean "this specific police".