You can't you have to get it from an event.
Pokemon pearl
Mew cannot be caught in Pokémon Crystal. Try to trade it from other games.
I don't think you can catch MEW without a action replay. But you can catch MEW2 in Pokemon Fire Red or Leaf Green. You CAN'T cach mew without an action replay, but you can trade mew-two from the other games to the newer ones
There is no way to go into space in any of the games.
You can't catch Mew if not through a official Nintendo event or using a cheating device.You cant catch mew. You can only catch mew if you hack.
It's impossible to catch mew in any pokemon game whatsoever (excluding glitches hacks gamesharks and cheatcodes) if you want a mew you'll have to do the mew glitch in the gen 1 games (no remakes). there are more explicit directions on how to do the mew glitch on youtube.
In Pokemon Red, Blue, and Yellow you are able to catch Mew through a glitch. Mew is available in other games such as FireRed and LeafGreen, but it was an event Pokemon. The event it over, and all you can do is wait for a future event, or trade with someone who has Mew.
as i know, in Pokemon emerald, ruby and sphere
i don't really think there's a way to catch mew in Pokemon platinum w/o cheating, but u can migrate it from the gameboy advance games if u have them.
you cant actually catch mew in any of the gba games unless you got the event thingy in emerald. if you have a game shark then you could get it but it wouldn't obey. so you cant really catch it...sorry
down the hierro island you can catch a mew in level 9.