When you first launch your game, you will see Squirtle, Bulbasaur & Charmander in front of you.
If you prefer Pikachu, just walk away...
Squirtle, Bulbasaur & Charmander will spring out in front of you again and again and again until
you walked for around 6mins before Piakchu will appear (At the 4th spring out).
Enjoy your Pokémon GO and look out for your safety while playing!!