When starting the game, the Professor will ask you to choose one of your starting Pokemon, a Squirtle, Charmander or Bulbasaur. But instead of choosing to catch one of them, turn off the phone. After a bit of time the phone will vibrate prompting you to chose one. Repeat turning off the phone. In total do this 5 times. On the fifth try you can finally capture a Pikachu as your starter.