Depending on which game you're talking about.... R/B or G/S/C??
For Pikachu:
R/B you can find them in the Viridian Forest, which is early in the game or the Power Plant later on.
G/S/C you can only find it on Route 2, or in Crystal, the Celadon Game Corner which isn't early on in the game...
For Pichu you have to breed from Pikachu or Raichu.