Actually, you have to save and enter the Verity Lakefront, pick the starter that you want and hope that it is shiny, like ElectricYellow45 said. If you didn't get a shiny, instead of turning the game off and turning it back on again, all you need to do if soft reset. You can do this on a normal DS by pressing and holding L, R, Start, and Select, and on a 3DS by pressing and holding L, R, and Start or Select. Soft resetting takes you back to the main menu of your Pokémon game, in this case, Pokémon Pearl.
Hope I helped!!