Viridian Forest is a maze-like area filled with wild Bug-type Pokémon and Bug Catcher trainers. The wild Pokémon you can encounter here include Caterpie, Metapod, Weedle, and Kakuna. Pikachu's Electric-type moves are not super effective against Bug-types, so rely on its Normal-type moves like 'Quick Attack' or 'Growl' to weaken them for capture.
There are several Bug Catcher trainers scattered throughout the forest. Battle them all to gain experience and money. pick up any hidden items you find. The forest is a good place to catch a Caterpie or Weedle if you want to evolve them into Butterfree or Beedrill later. The exit to Viridian Forest leads to the northern part of Route 2, which then takes you to Pewter City.