Instructions
Travel to Viridian Forest, north of Viridian City, or the Power Plant, found on Route 9.
Walk in the tall grass or inside the Power Plant until you are attacked by a wild Pikachu.
Use a combination of damage dealing moves and status changing moves to weaken Pikachu. A ground type Pokemon works well, as none of Pikachu's attacks will be effective.
Use your strongest Pokeball to capture Pikachu once he has been weakened. Pikachu is weakest when his health bar is red and he has been affected by a status-changing move at the same time.
Use additional Pokeballs if Pikachu breaks free. Once he is caught, you will be able to give a name to your Pikachu and he is added to your Pokemon lineup.