Use quotation marks around the exact words of a speaker. Use a comma to set off the quotation from the rest of the sentence.
Darius asked, "What are these gold plastic tubes for, Huan?"
Do not use quotation marks or a comma with indirect quotations.
Darius asked Huan what the gold plastic tubes were for.
Introduce a quotation with an attribution verb like "said" or "asked." The attribution can also interrupt or follow the speaker's words.
Huan replied, "Maybe they are part of a tuba or sousaphone."