Link: how to get Pikachu as your first Pokemon in Pokemon Go
At the start of the game, you are given 3 starter pokemons to catch: Squirtle, Charmander, and Bulbasaur.
However, there is a hidden Pikachu that you can catch other than those 3.
How to find Pikachu
Walk away from the three starter pokemons until your phone vibrates and the starter pokemons reappear.
Repeat step 1 for a total of 5-6 times. You will then see Pikachu among the starter pokemons.
Tap on Pikachu and catch him using your pokeball.