Kjotve will be the first Order Member you fight, but you won’t know he’s a part of the Order of the Ancients until later on. He is located in Kjotve’s Fortress in Rygjafylke.
You will fight Kjotve during A Cruel Destiny, which is one of the very first main story quests in the game. Because you fight him before you know about the Order of the Ancients, you won’t have any clues to investigate and uncover to locate his whereabouts.
He is one of the first bosses you’ll fight, so it isn’t too tough of a fight either, but for a detailed step-by-step guide, head over to the Kjotve the Cruel boss fight page.