To get Articuno you must go to the Seaform Islands in the Kanto region.
You go inside the cave and you do NOT go up the ladder, but you must go into that iced cave entrance.
You will appear in this cave made out of ice. So then you go through the cave. The video above the post will tell you how to get through.
Once you have gotten to Articuno you just walk up to him and click on him to go into battle.
It really isn't that hard to catch it. You could either use an ultra ball or since you are in a cave, dusk balls.
Articuno is an ice type and its attacks aren't that efficient,
so you could either use a fire type or an electric type.
You CAN get Articuno in Pokemon Diamond, Pearl and Platinum versions. I will put up a post on how to do it.
Well that's all for this post, bye!