Pikachu is not very hard to find and catch in the recently (although not fully) released Pokémon Go. But if you want him as a starter, you have to follow these three simple steps:
1.- Create a new profile, create your trainer character and talk to professor Willow.
2.- When he offers you your first Pokémon (Charmander, Squirtle or Bulbasaur by default), ignore that as many times as needed, meaning not touching on any of them with your finger.
3.- Without choosing any Pokémon yet, start moving, walk for a few meters and Pikachu will soon show up as a wild Pokémon you can catch with your Pokéball. He or she won't put much of a fight and Pikachu will thus be your first catch.
But in order to start a new game it's not necessary that you lose the old one. You can have two games on the same phone as long as you have access to multiple email accounts. Just go to the menu (tap the main Pokéball) and then go to options, exit. You'll then be back to the register screen, where you can connect to your old game or create a new one.