How do you get the legendary pokemon in heartgold?

Answer 1

artcono can be found at seafom islands acave next to blianes gym

zapdos can be found in the power plant after having the 16 badges

moltres can be found at mount silver

entai can be found any were around johto cheak the johto map in the pokegear

raiko can be found any were around johto cheak the johto map in the pokegear

Mewtwo is in Cerulean Cave, same as in Red/Blue/Yellow. But he'll only appear once you have the Johto and Kanto badges.