Answer
You have to beat the Johto regions.
Of course you will ask how . After you catch all Kanto Pokemon
and Mew , you beat the elite four ten times , and then prof Oak tell you that a road to Johto regions has opened under the victory road .
You can try to complete your Pokédex or battle at the Trainer Tower.
you must get the sapphire stone go to ...i think seven island and u will see a metal door with writing on it , use cut on the door and get the sapphire stone
Actually after you have beaten this part there is nothing really to do except catch them all.
Return to Viridian City to face the 8th gym leader, Giovanni.
You can find ditto in cerulean cave.
you can catch mewtwo at cerulean cave
mewtwo is in cerulean cave. the cave in in cerulean city, where misty's gym is. mewtwo in hard to catch, though.
i know one place is cerulean cave if u look hard enough
Cerulean Cave
Cerulean Cave
You can get Mewtwo in the Cerulean Cave.
Cerulean cave
mewtwo is in cerulean cave
Cerulean cave located near cerulean city.
inside cerulean cave near cerulean city.
The Cerulean Cave
You can find Mewtwo in the Cerulean Cave.
In Cerulean cave in Pokemon firered and leafgreen
You can find ditto in cerulean cave.
If you mean, "Wobbuffet", you can catch it in Cerulean Cave.
you can catch mewtwo at cerulean cave