The title may sound like a no-brainer. Fiction is 
full of dialogue; of course you need to know how to write it! That’s what I thought too.
Then I joined a critique group and discovered that one of the aspiring authors didn’t know how to punctuate dialogue. While judging for a writing contest, I read a submission with three-quarter-page-long paragraphs that contained the actions and dialogue of multiple characters all mashed together.
And when I started sifting through submissions here at Castle Gate Press, someone submitted a manuscript in all caps.
I no longer assume. Now I always recommend that new authors freshen up on the basics.
So here are the basics on writing dialogue:
BASIC DIRECT QUOTE – Set off a direct quotation with quote marks.
“We’re leaving now.”
**NOTE**
a) Punctuation stays inside the quote marks.
b) Punctuation for the direct quotation always remains inside the quotation marks.
c) Capitalize a direct quotation as if it is a sentence within a sentence.
QUOTATION FIRST – A period at the end of the quotation becomes a comma; other punctuation remains the same
“After the treatment we’ll run the tests again,” said Dr. Fowler.
“Are we ever going to leave?” asked her little brother.
**NOTE**
The punctuation stays within the quote marks.
QUOTATION LAST – A comma is needed to separate the quotation from the rest of the sentence.
The old man called over his shoulder, “I’m not dead yet.”
**NOTE**
a) The quotation ends with regular punctuation.
b) Punctuation at the end of the quotation stays within the quote marks.
SPLIT QUOTATION – The narration interrupting the quotation must be set off from the rest of the sentence with a comma before it and after it.
“After the treatment,” said Dr. Fowler, “we’ll run the tests again.”
**NOTE**
a) A set of quotation marks are needed around at the beginning and ending of both parts of the quote.
b) The comma after the first part of the quote stays within the quote marks.
c) The punctuation at the end of the sentence stays within the second set of quotation marks.
There you have it. Now you know that you know how to write dialogue!