Ok, you have to go to seafoam islands... Near Cinnabar island, but before you go remember to bring a pokemon that knows surf.
Bring Pokemon that know the HM-Surf, (Obtained by making it to the end of the Safari Zone in Fuchsia City) the HM-Strength, (Obtained by the owner of the Ember Spa, just south of Mount Ember where Moltres rests at the peak on One Island) and about five Ultra Balls to the Seafoam Islands. Once you're there, (I believe the northern one has Articuno. This one can't be accessed by Cinnabar Island, but Fuchsia City using HM-Surf) push the boulders using HM-Strength into the holes, but don't fall into them yourself. Take the ladders instead. The reason for pushing the boulders down the holes is to block currents on the bottom of the cave that are preventing you from Surfing. Once you push all the possible boulders down the holes, (I think that there are two that you can't push down a hole) use HM-Surf to Surf on the water where the currents used to be. Surf north until you reach a rock with stairs. On the western side of the rock lies Articuno. It's best to use an Ultra Ball on it while it's Hit Points (HP) are yellow and it's asleep. It's moves are Ice Beam, Mind Reader, Agility, and Mist. It is genderless and can be raised to learn Sheer Cold, a one hit KO move!
In seafoams island you need surf first