you grab the sapphire gem from the scientist and give it to celio on one island and after that you have acess to cerulean cave where you can grab mewtwo lv 70
After you beat the rocket warehouse go to talk to Celio and give him the sapphire. He will put it in his machine and this will let you capture Mewtwo. Then go to Cerulean city and do surf in the left...
In the warehouse in goldenrod (make sure you have beaten the radio tower, and have the basement key) go to 3F of the radio tower and open the shutters then battle to beat them