During Act 3 of the game you will get a quest called "Mine Massacre" from Hubert in Hightown (you will have to have finished the quests "The Bone Pit" and "Get back to work" earlier on in the game). You could also just travel to the Bone Pit and trigger it.
On the map you will have to go all the way to the south to a huge open area. The actual fight will alternate between fighting the high dragon itself and waves of dragonlings whenever he retreats. During the dragonling parts the high dragon will also shoot the occasional fireball towards your party. Keep an eye on their health.
The only thing to watch out for imo is when the dragon takes air and hurls your party to the ground. All in all the fight isn't too difficult though. You'll also get some decent loot including a High Dragon Firegland which you can bring to Hightown for the Herbalist's Tasks quest, an armor upgrade for Fenris, Dragons Blood (rare crafting resource) as well as some great equipment (like the chestpiece of the Mantle of the Champion set).