You'd capitalize it. The only time you don't want to capitalize a quotation is when it's part of a larger paraphrase, such as in this scenario:
Yesterday, Ms. Jones asked the reporters to "get serious about their assignments" and report back to her with the answers.
However, in this situation, it would be capitalized. The quote is either a complete sentence or an interjection which could stand on its own.
At yesterday's meeting, Ms Jones shocked the reporters when she said, "Get serious, folks! I need answers now!" The meeting ended immediately thereafter.
Comments
You'd capitalize it. The only time you don't want to capitalize a quotation is when it's part of a larger paraphrase, such as in this scenario:
Yesterday, Ms. Jones asked the reporters to "get serious about their assignments" and report back to her with the answers.
However, in this situation, it would be capitalized. The quote is either a complete sentence or an interjection which could stand on its own.
At yesterday's meeting, Ms Jones shocked the reporters when she said, "Get serious, folks! I need answers now!" The meeting ended immediately thereafter.
The Second is right. It is the beginning of a sentence so you need to capitalize it. "Why don't you stop that?" Is correct grammar.
The beginning of a thought should be capitalized. Thus, it should be "Why don't you stop that?"