you walk right in the cave island
its in seafoam island not cinnabar
meet blue, the gymleader on cinnabar island
Cinnabar Island.
Go east of Cinnabar Island to the Seafoam Islands.
when you are at cinnabar island keep on going east until you see a tiny island with a girl near it (picnicker) and go up the the ladder.
you have to have 7 kanto gym badges then find blue at cinnabar island
Go to Cinnabar island and talk to Blue. He'll agree to go back to his gym and battle you.
On Pokemon soul silver the cinnabar gym is located inside Seafoam Islands.
first it's not your rival and second the gym leader blue can be found on cinnabar island if you have all the other badges talk to him.
Well after you defeat 15 gym leaders in Pokemon HeartGold / SoulSilver go to Cinnabar Island and talk to Blue.
Blaine is located East of Cinnabar Island, his gym is where the Seafoam islands used to be. I think you have to talk to Gary/Blue on Cinnabar Island while you're there to face him later as well.
You head to Pewter City and battle Brock there gym leader! Then from there you head south to Virdian City. There gym leader won't be there so you keep on to Pallet Town. Then pass Cinnabar island and head to the seaform island and challenge the gym leader! Then after you defeat there gym leader talk to the Virdian Gym leader at Cinnabar island and then you go defeat him at his gym