Earlier this year owners of Pokémon X & Y as well as Pokémon Omega Ruby and Alpha Sapphire had an opportunity to pick up a special Mythical Mew 'mon as part of a limited distribution. The distribution was mostly through serial codes, with the Mew having the following details:
As it was limited to codes from stores some likely missed out, so The Pokémon Company is offering another opportunity. Everyone who's signed up to the Pokémon Trainer Club newsletter before 1st June will get that same Mew as a code that month; you can see the details and sign up here.
If you're already receiving the newsletter then all you need to do is wait patiently.