You can calibrate at any time, so that the exercises are in the right range for you. Select the feature by tapping the 
button and then the Continue button to start.
First find your most comfortable low note. To do this, sing as low as you comfortably can, and slide down with your voice, landing on the lowest note you can sing without straining your voice.
To finish up, find your most comfortable high note. To do this, sing as high as you can, and slide up with your voice, landing on the highest note you can sing comfortably without straining your voice.
That's it!
Next up: How the singing learning path works