How to get event Pokemon?

[PokeTony]

For example: http://www.pokemon20.com/en-us/mythical.html#may 
 Darkrai Is the Pokémon of Your Dreams!I live in USA so I will go to Gamestop, but what do I do at gamestop?
show ip with my 3ds and tell the guy at the front cashier or?

[Jaerri]

No, you don't need to have your 3DS with you (or the ID) just go there and ask for the Serial Code He then gives you a piece of paper where you can scratch the code free. 

"The Mythical Pokémon Darkrai will be available at participating GameStop stores starting May 1, 2016. Visit GameStop to pick up a code that will unlock Darkrai in your Pokémon Omega Ruby, Pokémon Alpha Sapphire, Pokémon X, or Pokémon Y game. Be sure to redeem your code in your game by August 31, 2016!"

There's even a link to a guide on the page! :D
http://www.pokemon20.com/en-us/mythical.html#may

[PokeTony]

yay thank you so much, is tyhere a link where they give more pokemon like this?

[DynastyK]

There's plenty to keep an eye out on. The arrows on the link Jaerri provided will show the other pokemons being distributed as part of the 20th anniversary, what month they're being distributed, and what method. Also keep an eye on their official site as well as their official youtube channel. And I'm pretty sure that someone here will at least alert others about event pokemons going around.