To get mew, first you must meet these requirements. Have at least 3 badges, have hm2 fly, and don't battle the youngster by bills house. He is above a tree. And don't battle the gambler thats right outside the underground path by lavender town. When you get at least that far, save it by the underground path by the gambler. Take one step in front of him and press start and then fly to cerulean city. Don't be alarmed that the pause button won't work because it's normal. Then battle the youngster I said before, but make sure you're one step away from him or the game will freeze. After you beat him, fly to lavender town walk to celadon, but you can't because when you do the pause menu will appear, but walk there anyway. After you get rid of the pause menu, mew will appear. Note: bring lots and lots of great balls or ultra balls.