After defeating Erika, you can go to any other Kanto gym leader you have not defeated yet. If you defeated all of them except for Blue, go to Cinnabar Island and show him the 7 Kanto badges you have obtained. He will go to the Viridian gym to await you to battle him.
After you beat the first 8 gym badges, you go to the elite 4 which is west of new bark town(surf in the lake next to New Bark town) then go north until you reach the victory road and then indigo league. after you beat the elite four, you can go back to Olivine city and take the boat to the Kanto Region to battle the next 8 gyms and then go on to beat Red(Red is actually Ash).
You get your 3rd badge in Veilstone city against Maylene.
In Diamond, Pearl and Platinum 7th gym is in Snowpoint Town.
the fourth gym is in celadon city at the bottom left of the city. the gym leader is erika. the badge is called the rainbow badge
go to celestic town in the middle there is house thing play a song on the badges and celibi wil apeer
Go to lavaridge town for your next badge.
sunyshore
You go to the Pokemon league through victory road.You have to win!!!
you got to go to veilstone and that's where she is.
You get your 3rd badge in Veilstone city against Maylene.
Go to hearthome city and beat the gym.
you go the the fourth gym in pastora and battle him
You should go to Canalave city
after you batle the odeburgh gym leader you go to eterna city.
go to Eterna city and defeat the gym leader(she has grass type pokemon).
hearthome i think
you cant
in eterna city