You need to go here and play the Breakout game: http://www.pokemon.com/us/fun-zone/online-games/play-to-befriend-a-pokemon_fg/
Complete the 4 levels and you will be assigned one of the Eevee evolutions. It's based on which icon you collected most of during the game. The icons fall down when you break blocks, for example a lightning icon for Jolteon.
You can't actually get the Pokemon just yet, it will be available in the Dream World after May 5th.