Curiosity Killed the Cat

I'm really curious as to how other people write. This semester I'm taking a fiction writing course at my university. Lately, we've been speaking about first lines and when to start the story. The advice my professor has given us is to start where there is material. For example, you have a forty year old man then you make him have a flashback to prom. If you were writing the story, would you make the character start off at prom or in his forties? Why?

Another thing we've been speaking about is first lines. You start out with dialogue, movement or people, that's a direct quote. How do you start a story?