Ian Castle tells us how to sing using a head voice in this video. The point where you cannot sing any higher is the point in which you need to use your head voice. This is a much thinner sound that feels lighter. To start an exercise to use your head voice, start with saying "aahhh" from a lower to a higher tone of voice. Do this several times using your entire mouth, not being scared to look silly. When you get to the point you can't reach your voice any higher, wait for a lower pitch and then start over again. Try this several times and with practice, you will reach an extremely high tone!