you get mew by having a pokemon that knows thief and its holding an ultra ball.go to
mistys gym and talk to her 5 times, talk to the girl beside her once then talk to misty
once then leave the gym and go to the pokemon center she will be there. Talk to her she
will ask you for a rematch. She will say come see me in my gym then go battle the e4 5
times then rematch misty
she will have 2 starmies and a seaking all on lv 50 and if u do this whole thing rite
she will have mew now use thief and mew will b in ur PC

Only one of the Legendary beasts can be caught, the one with a type advantage over the
starter (i.e., if you picked Bulbasaur, Entei can be found).

That L50 beast roams Kanto randomly after you have beaten the Elite Four and fixed Celio's
machine (found the Ruby and the Sapphire).

NOTE #1: Randomly means randomly. There is no way to ensure that both you and the beast
are on the same route at the same time.

year after you lave the s.s ane go on the walk way use serf to a truck on a island then
use sterangth on the truck there you will find mew