Instructions
Begin a new game, naming yourself and your rival as well as choosing your player character's gender.
Leave your house and attempt to enter the tall grass. Professor Oak will stop you and take you back to his lab.
Save before choosing your Pokemon. For quickest results, save while standing directly in front of Charmander's Pokeball.
Choose Charmander and name it if you wish. Enter the "Start" menu and choose the 'Pokemon" option. If your Charmander is a shiny, a star will appear next to its name and the sprite will be a golden yellow instead of orange.
Restart the game by simultaneously pressing "Start," "Select," "A" and "B" if Charmander is not shiny.
Choose Charmander again, once more checking whether or not it is shiny. If not, reset your game and try again until it is.