After emerging from Viridian Forest, you'll arrive in Pewter City, home to the first Pokémon Gym Leader, Brock. Before challenging Brock, it's highly recommended to heal your Pokémon at the Pokémon Center and stock up on any necessary items at the Pokémon Mart.
Brock specializes in Rock-type Pokémon, which are strong against Fire, Flying, and Bug types, but weak against Water, Grass, Fighting, and Ground types. Your Pikachu will struggle here, as Electric-type moves are not very effective against Rock-types. You'll need a different strategy.