First grab a book off a shelf, any book. Then close your eyes, flip it open and plant your finger on the page. You are welcome to swirl your finger about if you wish first. Where it lands is the first line of what one character says to another. Start your story there.
Sample: "used to ride a horse, which had feet that were almost human, the hoofs being cleft like toes." (Hen's Teeth and Horse's Toes by Stephen Jay Gould, p. 177)
"I say," Judson said, "he used to ride a horse, which had feet that were almost human, the hoofs being cleft like toes."
"Are you daft?" I said.
"Really. It's part of history," Judson sputtered. "I read it just this morning. Caesar rode such a horse."
I sat back on my heals and tried to look at the new born colt in this different light. The feet were not human, but there were in fact three toes on each hoof where there should have only been one solid toe.