Dark Souls' most famous covenant to become a Sunbro can be found within the Undead Settlement.
Once you make it to the first bonfire you have to traverse a little of the level until you reach a giant open area where Undead enemy NPCs are gathered around a huge fire by a burning tree. Defeat (or run way from) the mob and go north. A line of houses will be seen. Venture down the stairs and youll see a large stone archway with a wooden door to open to enter a house. On the right side of the room there will be a gap in the floor you can drop down and find the item to discover the covenant.