Drink a glass of water before you start to sing. This lubricates your vocal chords.
Adopt a comfortable singing posture. Sit or stand up straight and relax your neck and back. Breathe deeply through your nose and mouth.
Warm up your voice by singing the lowest note you are able to sing. The lowest note marks the bottom of your vocal range.
Take a deep breath and sing the next highest note. As you sing higher notes, transition from using your chest voice to using your head voice. Chest voice makes use of your abdominal muscles to produce low notes, and head voice relies more on your throat muscles.
Breathe deeply and continue to sing higher notes, gradually increasing the pitch, until you cannot sing any higher. Very high notes tend to sound shrill or like whistling.