You can actually go through the cave regardless of having the HM Flash. Simply use the little light they already give you and slowly make your way through. Also you use Flash if you really need it, to get Flash go to route 2 and use cut on the tree go north to a big building and talk to the scientist say Yes to his question you will get Flash. Use flash and now you can see through rock tunnel.
Rock Tunnel is located due east of Cerulean City, around the corner from the pokecenter.
Rock Slide is taught by a move tutor in Rock Tunnel.
in the rock tunnel, outside of lavender. there is some chick in a green outfit about half way through it.
use the move Flash (TM70)
i caught mine on the path from Cerulean city to Rock Tunnel
rock tunnel
Rock Tunnel is located due east of Cerulean City, around the corner from the pokecenter.
You take rock tunnel..........
Rock Slide is taught by a move tutor in Rock Tunnel.
in the rock tunnel, outside of lavender. there is some chick in a green outfit about half way through it.
On the way to rock tunnel,near cerullean city
Go east from cerulean city keep going until you reach water go south and into the cave this is Rock Tunnel.
Get to Lavender Town through Rock Tunnel (or Fly there, but the HM is impossible to get until you have already visited Lavender Town.)
in power plant, near entrance to rock tunnel, have to surf to it
use the move Flash (TM70)
Go through the rocky tunnel
you need to go from lavender town (via rock tunnel) west to route 8 and go through the underground path.