How to start the game with Pikachu

When the Professor prompts you to "Catch a Pokémon," walk away from Bulbasaur, Squirtle, and Charmander.

If you walk far enough away, the trio will respawn and your phone will vibrate. (We recommend walking along a sidewalk in your neighborhood to trigger this.)

Repeat step 4 3-5 more times, and a fourth Pokémon will appear: Pikachu!

NOTE: Polygon managed to make Pikachu appear after just 4 respawns, but it took me 5 to make this happen. Your mileage may vary.