Lifeguarding emergencies?

I was just wondering what some life-guarding would be because I love to swim and am think of becoming a lifeguard but I still have one more year before I can start Lifesaving classes at the pool. So one of my main concerns is how should a lifeguard react when someone goes off the diving board and hits their head on the bottom and is knocked unconscious? What would you do? How would you go about responding? What order would you do things in?