The song is called “Misty Mountains (Cold)” and was composed by Howard Shore, written by J.R.R. Tolkien, and is sung by Thorin and the dwarves in the film.
"Far Over the Misty Mountains Cold,
To Dungeons Deep and Caverns Old,
The Pines were Roaring on The Heights,
The Winds were Moaning in the Night,
The Fire was Red, it Flaming Spread,
The Trees Like Torches Blazed with Light.”