One of the most important elements of singing is a consistent and relaxed approach to breathing and breath control. A good singer always produces a strong, even tone and sounds relaxed regardless of how high or low the notes are.
To achieve this, you need to develop a method of breathing which will help you gain more control over the way you breathe when singing and give you a solid consistent approach which will eventually become automatic, enabling you to forget about breathing and concentrate totally on the music you are making.
A good way of developing your breathing technique is the use of visualization. When you breathe in, think of an inflatable mattress which fills automatically when you pull out the plug. This will help you equate breathing in with relaxation. When you breathe out, think of a tube of toothpaste being slowly squeezed from the end (not the middle). This will help you use your breath economically in a controlled manner.
It is important to develop the habit of breathing from your diaphragm muscle. As you breathe in, let the diaphragm relax downwards and allow the lungs to fill with air right to the bottom. Then breathe out slowly, squeezing gently from the diaphragm (like the tube of toothpaste) and see how long you can sustain your outgoing breath. The more control you have of your diaphragm, the easier you will find breathing when you sing.