To get a Mew you need 3 badges, and the moves Cut and Fly. Play through the game until you get to the point where you go up the bridge to Sea Cottage. Beat everyone but the guy in the rectangle, and continue playing. Fly to Celadon city and go through the underground tunnel. When you get out DO NOT MOVE! You must save your game now or else you won't be able to catch Mew. After saving, take one step and immediatly press Start. Normally the gambler would see you but he won't now. Fly to Cerulean city and go to the trainer you ignored earlier. Notice that the Start button won't work for a while. Walk above the trainer (don't talk to him) and beat him in a battle. The Start button will work again. Fly to Lavender town and walk away via the west exit. The menu screen will automatically pop up. Press B to cancel it, and you will be attacked by a level 7 Mew, with the only move it has so far, Pound.