Learn Italian: La benzina costa molto questo mese.

In this sentence, molto is acting as an adverb modifying the verb costa (it tells us how much it costs). Adverbs never change their endings. If it were an adjective describing a noun instead—like "a lot of petrol"—it would match the feminine noun and become molta benzina.

Does benzina just mean "gasoline" or "petrol"?

Exactly. Despite looking a lot like the English chemical term "benzene", benzina is simply the standard, everyday Italian word for the fuel you put in a regular car.

Why is there no preposition before questo mese? Shouldn't it be "in questo mese"?

In Italian, time expressions using questo (this) or quello (that) usually drop the preposition. You can just say questo mese (this month), quest'anno (this year), or questa settimana (this week) directly.

Why do we need the article La at the start of the sentence?

In English, we usually leave out the article when making a broad statement about a substance (we just say "Petrol costs a lot"). Italian, however, requires a definite article (il, la, etc.) when talking about general categories, concepts, or substances.