This is one of the most often asked running questions. Runners who ask this question are starting their runs too fast.
Breathing is a very natural well regulation function of the body.
If you feel you are breathing too hard, it is your body telling you to slow down. If you start your runs and walks easy and allow your body to warm-up, you will never need to ask the question. If you start too fast, you will be breathing heavy, trying to get more oxygen into your body and to get rid of the carbon dioxide. This is a very uncomfortable feeling and part of the pain associated with running. There is a natural pattern to breathing.