You can't. The only way is to use Action Replay but I wouldn't risk corrupting my file or ruining your game with that junk. Otherwise you can catch him in Pokemon Leaf Green I think and then migrate.
In the original Red and Blue versions Mew is legitamely obtainable through a glitch. The addition of Mew was a last minute choice by the creators of the game so it's no wonder why something as strange as a bug in the game makes it appear. To find Mew you need to do several things: First you need Fly, Second you need to skip two important trainers: One Youngster that is facing upward on Route 25 he has a Slowpoke, and one Gambler facing upward toward the Underground Path leading to Celadon City on the route west of Lavendar Town, Third you need to save before attempting this (this is a very good move to do) and save in the Pokemon Center in Celadon City, Fourth comes the hard part: Go to the Underground Path that leads to the route west from Lavendar Town then save by the exit then leave, right below is the Gambler we need to talk to however we will not be battling this trainer instead do the following: Walk down and quickly hit Start to open the menu and if your quick enough the menu will appear and now open up the Pokemon screen and choose your flying pokemon to Fly to Cerulean City and if all goes well the Gambler will see you but not battle you, now walk all the way up to Route 25 and have the Youngster battle you (HE MUST WALK UP TO YOU so make sure your a step above him) and then defeat him, finally use Fly again and head to Lavendar Town and now head west and the menu should open up by itself when this happens press B or hit the Exit button to activate a battle and the pokemon that should appear is Mew. It will be level 7 and hopefully you brought Ultra Balls or the very least duplicated Master Balls. Obviously at this point capture the pokemon and you will be the proud owner of an normally unobtainable legendary pokemon!
Yes, you know the Fuego Ironworks just North of Floaroma Town? Well it Randomly pops up there but you need the National Pokedex.
yes you can
No, you cannot.
no now shhh im trying to get to sleep
in Pokemon yellow, blue, red, fire red, and leaf green you can find mew-two in cerulean cave after you have beten the game, but for mew at the moment you can not get it without cheats...
mew is on island 9
Mewtwo, yes, if you have a FireRed or LeafGreen with Mewtwo.
No, you cannot.
you can't
no now shhh im trying to get to sleep
you have to trade from emerald
no you dop
in Pokemon yellow, blue, red, fire red, and leaf green you can find mew-two in cerulean cave after you have beten the game, but for mew at the moment you can not get it without cheats...
you can not get mew without the mew glitch but if you go to the end of cerulean cave you can get mewtwo
mew is on island 9
You can't get mew in firered or leafgreen.
mew and mew two
transfer from emerald
you can get mew and mewtwo by migrating it from Pokemon fire/green