How do you go in to the cave to get mew two

you have to beat the elite 4 the first time and then catch at least 60 species of Pokemon and take it to oak and he will upgrade it then go to the 3 islands and start helping celio and he will give you the rainbow pass to go to all seven islands and then finish off all of that and then go back and fight the elite four again and all of their Pokemon will all be 10 levels higher and with new/evolved ones and after that you are able to go to the cave