U must finish all gyms before u can get there
You can catch Articuno on seafoam island whenever you don't have to beat the Pokemon League.
In the Seafoam Islands, east of Cinnabar Island. You need HM Waterfall, though.
If you don't want to go through seafoam islands there is a shortcut if you want to take it, Fly to pallet town and go south use Surf and continue south until you reach Cinnibar island guess what you didn't have to go through seafoam island!
seafoam islands
Seafoam islands
I assume you're talking about Articuno. To get Articuno you have to go to the Seafoam Islands, which is on route to Cinnabar Island.
Inside Seafoam Islands.
Seafoam islands.
you get articuno in seafoam islands
seafoam islands
Seafoam Islands.
Seafoam island is east of cinnabar and Articuno lives there.