Once your Necromancer reaches Lv25 in Diablo 4, you'll get the quest "Necromancer: Call of the Underworld".

Find a Shrine to Rathma along the norhtwestern edge of Sarkova Pass, where a shade name Maltorius will instruct you to collect Unbroken Bones before tasking you with freeing some trapped souls. Follow Maltorius's instructions and you'll end up with your very own Golem!