Before you can challenge Misty, you'll encounter your rival, Blue, on Nugget Bridge (Route 24, north of Cerulean City). He'll challenge you to a battle. His team will have evolved since your last encounter. Be prepared for his Pidgeotto, Rattata, and Kadabra (if he evolved his Abra). Your Pikachu will be useful against Pidgeotto.
After defeating Blue, head back to Cerulean City. Make sure Pikachu is at a good level (around Level 20-25) and has strong Electric-type moves. If you caught a Bulbasaur from the NPC in Cerulean City (if your Pikachu is happy enough), it will also be very effective against Misty's Water-types.