"How did Harry's parents die?"

Ok guys, this is just sad. It says right in the first book that Harry's parents were murdered by Voldemort. Harry was fed a lie by his aunt and uncle. They told him his parents were killed in a car crash. As to why Voldemort killed his parents..... Voldemort actually was just trying to kill harry because Severus Snape informed him of a prophecy saying that a boy born in the end of July and whose parents escaped three times would defeat him. He believed that was Harry. His parents died trying to protect Harry.