It depends on what level you are talking about learning to breath.
For a first-time swimmer, doing bobs (submerging the face and blowing bubbles, then coming up for air, repeatedly) helps. This reinforces the practice of blowing out your breath underwater and not holding your breath.
For beginners, being comfortable in the water. Might sound simplistic but a fear of the water (even a slight apprehension) makes breathing difficult.
If you are talking about the front crawl (freestyle), the one hand stroke drill helps to emphasize correct body positioning while breathing and practices breathing on both sides.