It's actually pretty easy to find these sorts of quotes. All you have to do is treat the specific chapters like a word search puzzle and skim them until your eyes fall on the either the words, ash, shadow, darkness, dark, or death. Skimming a text in search of something is actually a critical skill to learn for higher education, as we really can't complete large essays, such as college-level research papers, without it. The words ash, shadow, darkness, and dark are especially used frequently by F. Scott Fitzgerald in The Great Gatsby as he uses them to symbolize moral corruption and decay.
Just skimming the first chapter alone, we can quickly spot two relevant quotes you're looking for. The most obvious ones are towards the end of the chapter. Our narrator notices Gatsby looking looking out across the water at a single green light, the same light on Daisy's dock. The narrator describes that Gatsby "stretched out his arms toward the dark water in a curious way." The narrator further adds that when he turned to look at the light and looked back at Gatsby, Gatsby had "vanished, and [Nick] was alone again in the unquiet darkness" (Ch. 1). We know that these lines have importance because they symbolize the hurt, torment, and darkness Gatsby feels in his heart due to losing Daisy when he was still a poor man. Also, since darkness can symbolize sin or immorality, these lines also symbolize Gatsby's moral corruption.
Earlier in the chapter, when we meet Jordon Baker, Nick realizes why she looks familiar. It is because she is a professional golfer whom he has seen in "many rotogravure pictures of the sporting life at Asheville and Hot Springs and Palm Beach" (Ch. 1). It's of no small significance that Fitzgerald chose to refer to the name Asheville. Asheville, along side Hot Springs and Palm Beach would represent the relaxed, wealthy class of the era. Later in the book, we see how Fitzgerald uses the word ash to also symbolize moral decay. Therefore, mentioning Asheville is the first symbolic reference to moral decay.