Brock uses a Geodude (Level 10) and an Onix (Level 12). Both are Rock/Ground types. Pikachu's Electric attacks will do minimal damage. Your best bet is to catch a Nidoran♀ or Nidoran♂ on Route 22 (west of Viridian City, accessible after delivering Oak's Parcel) and train it. Nidoran learns 'Double Kick' at Level 12, a Fighting-type move that is super effective against Rock/Ground types.
Alternatively, you can catch a Mankey on Route 22. Mankey is a Fighting-type Pokémon and learns 'Low Kick' at Level 9, which is also super effective. If you didn't catch one earlier, you can also trade a Spearow for a Farfetch'd in Vermilion City later, but that's a bit far. Training a Nidoran or Mankey is the most common and effective strategy for new players.