Sing with your mouth open while keeping your embouchure and jaw relaxed.
Drop the bottom part of the jaw to allow your soft palate to rise up. By dropping your jaw, you will notice that your tongue naturally rises. This is crucial for developing a clean, soft sound.
Relax your throat and allow your tongue to come forward slightly. The tongue shouldn't leave your mouth, but you do need to bring it forward slightly to allow air to travel through your throat easily.
Stand straight with your shoulders back, your chest should be high to open up your diaphragm and allow air to come freely.
Breathe into your diaphragm when taking a breath. Avoid the tendency to breathe shallowly into the chest.
Sing long tones to help develop your vocal tone. The long tones should last at least four seconds. These will help to improve your voice over time and provide you with a solid warm-up.