Use commas, or sometimes a colon, to set off direct quotations that are complete in themselves:
The Unabomber’s manifesto begins, “The Industrial Revolution and its consequences have been a disaster for the human race,” and goes on to condemn modern society.
But don’t use a comma before a quotation that’s part of the flow of your sentence:
Kaczynski goes on at length about the “industrial-technological system” and what’s wrong with it.
Learn more about punctuating quotations.