Learn Turkish: Sokak şimdi çok sessiz. - The street is very quiet now.

You are right that çok often means "a lot" when used with verbs, or "many" when used with nouns. However, when you place çok directly in front of an adjective like sessiz (quiet), it acts as a degree modifier and translates to "very".

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

In Turkish, there is no standalone word for "am", "is", or "are". For the third-person ("he", "she", "it", or nouns like sokak), the "is" is simply understood. You don't need to add any personal suffixes to the end of the sentence to show it.

Does the word sessiz come from ses (voice/sound)?

Yes! The suffix -siz (which changes to -sız, -suz, or -süz depending on vowel harmony) means "without" or "-less". So ses (sound) + -siz literally means "soundless", which is how Turkish expresses "quiet".

Could I put şimdi (now) at the very beginning of the sentence instead?

Yes, Şimdi sokak çok sessiz is also perfectly correct and natural. By putting şimdi after the subject (Sokak), the speaker just puts a little more focus on the street itself as the main topic of the sentence.