This is a common question, and usually what the person asking wants to know is: Do I start with an idea, a character, an image, a plot? Do I start with a setting? A theme? Or perhaps a conflict?
The truth is, while a novel must contain all of these elements, it doesn’t much matter where you start. Perhaps a better question is: How much do I need to know about my story before I start writing it? And the answer is: That depends on what sort of writer you are.