Instructions
Get a Pikachu in your party. This can either be done through trade with another player or through the traditional find-and-catch method. Pikachu are common and can be found easily in the Trophy Garden area off Route 212 behind the Pokemon Mansion. When capturing a Pikachu, it will be weak to Ground-type attacks, while resistant to Flying, Steel and Electric.
Travel to Hearthome City. It is in the south-central part of the map of the area (called Sinnoh) at the junction of Routes 208, 209 and 212.
In Hearthome City, travel to the north part of the city and look for one of two gates leading to a place called Amity Square. There will be two entrances in the north part of Hearthome, one one the west side of town and one on the east.
Go in one of the two gates and through the gatehouse room. Before you go up to the girl sitting at the desk, make sure your Pikachu is in your party and set as your first Pokemon.
Talk to the girl at the desk. She will tell you that you can take a walk with one of your "cute Pokemon." Tell her "Yes," and your Pikachu will start to follow you.