Pokemon league (or if u did what i did, u can go beat red first then red.) if you did the Pokemon league then they would have more higher level Pokemon and other Pokemon from other regions. check serebii.net for more info about it.
You need to complete the gyms and the elite four in johto first
you need to beat all gyms, the E4 twice and red.
You have to beat all 16 gyms.
catch all the pokemon.
there is another region in the game that is red version and you have to beat all the gyms.
You have to beat all 16 Kanto gyms, then talk to professor oak.
beat all 16 gyms and go talk to prof. oak in paett town
after you beat all 16 gyms, go to the bottom left exterior of the power plant.
He is inside Mt. Silver which you can reach after beating all 16 gyms
Just beat all the gyms. The first one is in Violet City
Beat all the kanthan gyms and go to mt.silver and beat red
Blue is the viridian gym leader and red is on top of mt silver once you beat all the kanto gyms