Learn Russian: Это не улица, это площадь. - This is not a street, this is a square.

In Russian, you usually drop the verb "to be" (is/are) in the present tense. To make a negative statement, you simply place the word не (not) directly in front of the word you are negating. Here, it goes right before улица to mean "is not a street".

Since both улица and площадь are feminine nouns, why do we use the neuter word это instead of эта?

When это acts as a standalone subject meaning "this is" or "it is", it always stays in its default neuter form, no matter the gender of the noun that follows. You would only use the feminine эта if "this" was an adjective attached directly to the noun, like in эта улица (this street).

Is the comma required between the two halves of the sentence?

Yes. Just like in English, the comma separates the two independent ideas ("this is not a street" and "this is a square"). It also tells you to make a slight pause when speaking.

How do I know the gender of площадь since it ends in a soft sign (ь)?

Nouns ending in a soft sign can be either masculine or feminine, so you just have to memorize their gender when you learn them. площадь happens to be feminine. The soft sign itself just tells you to pronounce the д with the tongue pressed higher up, making a softer sound.