You've talked to people your entire life. Sometimes you've explained to them, and vice versa. Pattern your dialog around the conversations you've had. But here is the kicker.
You have to keep the dialog consistent with the character you've created. You can't create a macho soldier who is a walking machine of destruction and have him discussing the contrasting colors that bring a room to life. It doesn't work.
Figure out what the goal of the story is, and if the dialog is intended to move the story forward, or is background to give a better feel of the characters involved.
You've made facial expressions, and seen them all your life. So when a regular woman hears something she doesn't believe, roll her eyes as she responds, include the expression.
I create the scene in my head. I play it backwards and forwards. I see if the dialog fits the people. I see if the expressions feel normal, believable, and accurate to the situation. People in the middle of a shootout aren't going to be discussing the upcoming birthday party for a friend. They may argue, they may exchange information based upon points of view, they may make a joke (graveyard humor) but they won't be swapping Honey Mustard recipes.
You've had a million conversations, from hello and how are you doing to serious conversations with points to be made. Have them in your head, both sides, three sides, how ever many there are. Create your characters first, and imagine how they would speak. Love your characters, because then you can do them the most justice in your writing.
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