Articuno, along with Zapdos and Moltres, are the Legendary Birds that are only obtainable in the Pokémon Silver version. You can find them in the "Lake of Rage" area.
Here's how to catch them in Pokémon Silver:
1. Complete the game: You need to have finished the main storyline and obtained the National Pokedex.
2. Obtain the Silver Wing: This item is required to catch Articuno. You get it from a man in the "Seafoam Islands" after completing the main story.
3. Head to the Lake of Rage: It's located in the northeast region of the map.
4. Defeat the trainer: A trainer is guarding the lake. You have to defeat them to access the area.
5. Face Articuno: After clearing the path, Articuno will appear. It's a powerful Pokemon, so you need to be prepared for a tough battle.
You can trade Articuno with a friend who has Pokémon Silver. This is the only way to get the Legendary Birds in Pokémon HeartGold.