Proper posture improves the quality of sound. Stand up tall with the shoulders back, the abdominal muscles pulled in and the tailbone pointed down. Keep moving to avoid tension in the body; shift the weight in your legs, or take a couple of steps.
A professional sings from the diaphragm, which is located just below the lungs. To reach the diaphragm you need to breathe deeply, expanding the air deep into the lungs, which causes the stomach to rise and fall. When singing, maintain the flow of breathe, and exhale slowly.
To open the whole mouth, relax the lips, and hinge the jaw down but not forward. To create good resonance, keep the roof of the mouth high and arched and the tongue flat at the bottom of your mouth, except when enunciating. Try imagining an egg at the back of your throat.
Study the music, the words and their meanings. Understanding the music helps to sing with feeling, and knowing the words helps with proper enunciation.