Open new paragraph everytime you have a different speaker. The proper format for writing dialogues is that every new speaker in a sequence must be assigned a new paragraph. When presenting alternating dialog between two speakers, assign each speaker a new paragraph as they take turns to speak. New paragraphs let readers keep pace with the changing speakers and keep the page from becoming cluttered.
Devise a uniform system for marking dialogue. The most common way of marking dialogue within a novel is with the use of open and closed quotation marks. Open a quotation mark, write the dialogue and close the quotation mark once the speaker is finished talking. The entire content inside the quotation designate the exact words of the character and not the narrator or the author. If you decide to use an alternative system like dashes you may begin the dialogue with a dash and end with a comma or a period.
Mark the speaker's closure with a comma or period. The end of a character's speech does not have to coincide with the end of the sentence. If it does, you insert a period before the closed quotation marks. If the sentence continues, for instance, if you want to qualify their speech by writing, "said Andrew" afterwards, insert a comma before the closed quotation marks and insert a period after the words "said Andrew."
Use meaningful dialogue labels. Don't always get stuck with conventional labels like "said Andrew" or "said Julie." Use alternate labels from time to time, or omit labels altogether to keep the dialogue smooth and meaningful.