Instructions
Play through the game until you receive a Togepi. This occurs right after you defeat the Violet City Gym.
Capture a Ditto, only found in the Safari Zone. Catching Ditto is just like catching any other Pokemon in "HeartGold." While you can breed Togepi with other Pokemon, this is the only way to guarantee you receive another Togepi, as you cannot create more Ditto.
Travel to the daycare south of Goldenrod City, and give your Ditto and Togepi to the old couple. Walk around outside for a few minutes then speak to them again, and they will give you an egg.
Walk around with the egg for a while until the game tells you it will hatch, giving you another Togepi.