After defeating Misty, head to the house in the northwest of Cerulean City. A Team Rocket grunt has stolen a TM. Defeat him to retrieve it. You'll then need to head east onto Route 9. This route has several trainers and leads to the entrance of Rock Tunnel. Make sure you have a Pokémon that can learn HM05 (Flash) before entering, as the cave is pitch black without it. Flash can be obtained from Professor Oak's aide on Route 2 after catching 10 Pokémon.
