The fourth limb of yoga, according to the Yoga Sutras of Patanjali, is pranayama or breath control. I feel that this is perhaps the most important limb, because it is the key to linking the body and mind. There is movement in breath control – movement of air, movement of lungs, movement of ribcage and abdomen, but there is also stillness and focus in between breaths. There is awareness in the here and now during control of the breath. It is this stillness and this awareness that make the practice of asana truly yoga.
There are many different formal styles of pranayama, all with different techniques and purposes. They can have a profound impact on your state of mind and your physiology, and should be treated with caution. It is thought that by controlling your breath, you are also controlling the movement of energy in the body. If you would like to practice pranayama, I would recommend seeking out a teacher who is properly trained in the techniques and impacts of various types of pranayama, so that they can guide you safely through the practices.
A great place to start in the meantime, is with observation of the breath. Without trying to change the pattern of your breath, simply close your eyes, or focus them on something still. Notice the feeling of the breath moving in and out of your body. Tune in to the movement of your lungs, your ribs, and your belly. Once you can feel the movement there, let your awareness travel further from the centre of the body, and notice any sensations. This is the foundation for pranayama work. Before you control the breath, you must be able to tune in and become aware of it.