This is a very common exercise. It is comprised of a 5-note scale.

1. First, you sing the scale regularly, in the major key.
2. Next, you take the third note down a half-step and sing the scale in the minor key.
3. Then, to further vary the scale, you would sing it chromatically by taking each note down a half-step.

Try each of these all the way up to the top of your range before moving on to the next.