With wind instruments, good breathing is essential to good sound. Your breath supports everything about your playing, from range and stamina to volume and pitch, and it all starts with posture. Sit up straight but comfortable. Bending your body obstructs air flow and keeps your lungs and diaphragm from expanding properly.
From there, breathe as deeply as possible, aiming to fill up your chest, lungs, and stomach. Avoid breathing solely through your nose, which won’t provide enough air. When breathing out try to blow the air out from the back of the throat for a warmer, fuller sound. It’s the difference between blowing on some hot soup and blowing on your hands when they’re cold. The second type is optimal.