How does a shy person negotiate the price of a car?

I am super shy, and probably going to buy a car today. They are asking $5600 and the KBB "Fair" price is $5600. So no matter what, I think the price will be fine. It's negotiable, though, so I should ask for less. How do I do it? What do I say? How low should I offer at first? It has a few problems, like a window that doesn't go down, so I could use that to ask for lower.