In spoken language, the five basic vowels are E, A, I, O and U. In singing these are modified to the sounds EE, AY, AH, OH and OO. Run through these sounds several times on one pitch and become aware of the way the shape of your mouth and position of your lips changes as you move from one sound to the next.
As you sing these sounds, your tongue should remain relaxed in the bottom of your mouth and your jaw should also remain relaxed. Aim for an "open" sound and feeling as you sing each vowel. Once you are comfortable with all the sounds on a single pitch, try the following exercise, which descends through the major scale. It is important to do this exercise slowly at first.