Jeez. Alot of you guys are getting the town names mixed up which can be a little confusing when someone needs help.
First of all, this should have went in the stickied topic:
HG/SS Help Thread which is the first thread on the entire HG/SS Section.
Secondly. You need to complete the Power Plant mission.
Go to the Power Plant, learn about the stolen part, then go to Cerulean Gym.
The Rocket guy will run out, go after him, he'll be on Nugget Bridge.
Beat him there and he'll reveal where the part is, in the Gym.
Get the part and return it to the Power Plant manager.
Assuming that you've said you completed Cerulean's gym, that must mean you've already completed the Power Plant mission.
If that is so, return to Fuschia and go up Cycling Road to reach Celadon.
Or go to Saffron and exit through the left gate into Celadon.
There, you can find the Celadon gym behind a load of trees in the bottom half of the city.
Beat Erika there, her type is Grass, but you already knew that anyway.
If you have beat the Power Plant mission, be sure to:
Go to Lavender Town and talk to the Radio Station manager there.
He'll configure PokeGear to be able to pick up Kanto radio stations.
Go to Snorlax. Go to PokeGeare and manually drag the little ball thing on the radio signal to the top middle of the radio signal. This should tune you into PokeFlute.
Then just hit A on Snorlax and he should wake up and battle you.
Capture/Faint Snorlax and proceed through Diglett's Cave.
When you exit at the other end, you'll be on a route between Viridian and Pewter.
Make sure you have a Pokemon with CUT.
Cut your way out onto the route.
Go up for Pewter City and Brock's gym.
Or go down through Viridian Forest to reach Viridian.
Viridian's gym leader ain't home.
So you can continue going down to Pallet Town.
From Pallet Town, you can surf down to Cinnabar.
You'll find Blue there.
Talk to him and he'll tell you that you aren't ready to fight him yet.
Surf right from Cinnabar to go to Seafoam Islands so you can face Blaine, the old Cinnabar Gym Leader. He has fire types, obviously.
Once you've beaten 7 Kanto leaders, go back to Cinnabar and talk to Blue again.
He'll say you're ready and to meet him at his Gym in Viridian.
Go there, beat him and then go to Pallet Town to see Oak.
God, that was long. But detailed and to the point.
Sure hope that helps you.
Next time, use the
HG/SS Help Thread for these sort of questions.