Instructions
Purchase some pokeballs and head towards Viridian Forest. Walk around in the grass until a wild Pikachu appears. You'll need to find a Pikachu that is holding a light ball. This is rare and may take some time, but you increase your odds of finding one by catching multiple Pikachus. You should make sure to have at least one Pikachu of each gender. You can also use a Pokemon who knows the moves "Frisk" and "Thief" to make the process easier, though it's not necessary. If you have a Pokemon steal a light ball from another Pokemon, you must attach it to a Pikachu.
Navigate your way to the breeding center on Island 4. Place both your male and female Pikachus in the center, making sure at least one of them is holding a light ball. You can also breed your Pikachu with a ditto to achieve the same results.
Ride your bike around the island until you notice an egg in the breeding center. This can take between 3 to 30 minutes depending on how much your Pokemon like each other. Once an egg appears, talk to the old man and he will give you it to hatch. The egg will hatch into a Pichu that knows "Volt Tackle" after you walk 2,560 steps.
Evolve the Pichu by raising his happiness points. This can be done by keeping him in your party and having it battle other Pokemon. You can also give him items to speed up the process, such as HP up, protein, and zinc. Once the Pichu evolves into a Pikachu, it will know volt tackle.