On Writing: How To Talk

What your characters say is just as important as what they do. In fiction, dialogue is action. It reveals characterization, it moves the plot, it provides information. Well, it's supposed to.

Listen to people.
Listen to your family, your friends, people on the bus, people on the train, people at the zoo, people at the grocery store. Listen to what they say and how they say it, what words they use. Pay attention to your assumptions of them based on their speech, to how their words relate to their age, sex, and apparent economic position. Listen to how they interact with others. The absolute best way to learn how people speak is to just listen.

Learn how to properly format and punctuate dialogue.
Because it's action and because our eyes slide so smoothly over well-formatted and properly-punctuated dialogue, we as readers don't really notice it. This is a good thing. So learn the mechanics of it. Learn where to put action in relation to dialogue (admittedly, something I still struggle with). Learn when to start a new line, where to put commas and periods and quotations, learn how to use multiple sentence-ending punctuation (like ellipses and the interrobang).

Learn how to use dialogue tags.
He said. She said. He whispered. She hissed. Learn where and how to use these attributes and learn how to avoid "creative dialogue tag syndrome." (Stella Deleuze posted a series of excellent articles on the subject of dialogue attributes recently. Here's the first post.)