I think you need to beat 7 kanto gym leaders to fight him.
you find Giovanni in silph co.
The gym leader is Giovanni, leader of Team Rocket. You have to come back to Virdian last as he is the Eighth gym leader. THE LAST ONE!
you have to have beaten all the other trainers and also Giovanni the rocket team leader in the silph company.
you can find it in the ciann city pharmacy
Blue is on Cinnabar Island. But to be able to fight him, you must find Blaine the old Cinnabar gym leader. He is in the seafoam islands. Beat him, and then talk to Blue again. He will then return to his gym, where you can fight him!
In Blackthorn City.
ecruteak city
In celadon city ^Technically, that is true for the first generation of Gold, Crystal, and Silver. But in heart gold and soul silver, the fourth gym leader in Kanto is Misty (cascade badge) if she is not in her gym in Cerulean City, then you can find her up on Route 25. ~Kt
once you get 15 gym badges go to cinnabar island find this person near a place where you use rock climb he is facing the wall talk to him he will tell you that he will be at his gym at viridian city go there and battle him..........
Virdian forest
You can only find it in the virdian forest in kanto or just trade through global trading system in goldenrod city.
Go to the place where the people are saying that there are wild Pokemon, Professor Oak will come and choose your Pokemon and then get there and at the end of the grasses you will find Virdian City.