Here's what you can do to encounter him:
* His House: You can find Bill at his house in New Bark Town. He's usually there, and you can talk to him to get some information about Pokemon.
* Celadon City: Bill is also in Celadon City, specifically the Game Corner. He's in the back room of the Game Corner.
* National Dex: Once you complete the National Pokedex, you can find Bill at his house in New Bark Town. He'll be there with his wife, and you can talk to them both about Pokemon.
Remember that Bill's appearances in HeartGold are limited to these locations. He doesn't appear at the Pokemon Center like he does in other games.