How do you get a legendary bird?

For FireRed and LeafGreen:

If you want an Articuno, go to the Seafoam Islands, which you can get to by way of Fuchsia City. Surf southward until you find two caves.

If you want a Zapdos, go to Lavender Town, and then head north. Surf on the river until you see a weird building.

If you would like a Moltres, go to One Island. Walk along the beach and surf north, and eventually, you'll find a place called Mt. Ember.

For Red, Blue and Yellow, Moltres is found on Victory Road.