Drive up to a pedestrian and press [Horn] to honk your horn. A menu screen will appear and give you choices of finding a shop, a race, and just forgetting about talking. Choose to find a race and a car will appear on the map. Go to that area. When you get near to the car, honk the horn. The driver will say 'You honk your horn at me punk?'. Press [Enter] and go to the bottom right hand corner. Set what you want to race for (prestige or money).