Physiologically, abdominal breathing is more effective than breathing shallowly into our chest, which is what many people do. Shallow breathing is a result of the stress response in the body. Additionally, in a culture that promotes a flat belly as desirable, some people hold their stomach muscles which interferes with deep breathing.
When we breathe deeply into the lungs, the diaphragm lowers and as a result, the abdomen lifts and spreads. This kind of breathing is more calming to the nervous system and induces the relaxation response in the body. It is also more efficient in increasing oxygen as you inhale and removing carbon dioxide as you exhale. Deep breathing can help stabilize or reduce blood pressure and slow the heartbeat.
This technique takes the breath in through the nose and out through the mouth. Once you get more comfortable, you can begin to take the breath in and out through the nose.
Before you practice this technique, I recommend you start with this simple breath awareness as a first step to finding your breath.