Luigi is a character from the Mario franchise, not Pokémon. He's a fictional character that doesn't exist in the Pokémon universe.
You can, however, get a Pikachu that wears a Luigi hat! Here's how:
1. Beat the game: You need to have finished the main story of SoulSilver.
2. Visit the Game Corner: Located in Goldenrod City, this building has a variety of games to play and prizes to win.
3. Play the slot machines: Keep playing until you win a special prize: a Luigi Hat.
4. Head to the Pokémon Center: Once you have the Luigi Hat, go to the Pokémon Center.
5. Talk to the girl: She's near the front desk, and she can give you a Pikachu wearing the Luigi Hat.
So while you can't get Luigi himself, you can get a cool Luigi-themed Pikachu!