First of all, it is necessary to understand what is the head voice that all singers talk about and where this head voice comes from. There are essentially two kinds of voice that you can produce through your vocal glands or your throat.
One is the chest voice which comes from below your vocal chords, literally, because we take in and leave the breath right from the chest or the area of the lungs.
Another kind of voice is a more compressed form of the chest voice because the air does come from the chest or lungs, but seems to be resonating in the area above your vocal chords mainly. This area is the upper part of your throat and the area in and around your mouth. Maybe that is the reason it is called the head voice, because the head is the area that is visible above your neck and the voice is produced from that very area.
You will understand how this works if you observe the condition of your mouth when you are yawning. Open your mouth and try to fake a yawn. You will realize that your mouth opens up wide and clear and your tongue is pushed down to the base of your mouth. This leaves the upper part of your mouth to be used like an organ, where the air can resonate and create a sound with a deep and clear ring to the ears. This very sound is referred to as the head voice.
Now, how to sing in your head voice is easy. Open your mouth wide like you would if you were yawning. Now your tongue should go down automatically as a reflex action. Try to sing a few notes like “Do Re Mi…” You will get the hang of it very easily and you will also realize that the sound of the notes is almost vibrating in your head.
Some vocalists confuse the head voice with the falsetto or the half-voice. However, you should know that head voice is very different from the falsetto in the way they are created by your vocal chords. The falsetto is almost like your voice cracking while singing some line and creating a very sensuous effect if done correctly, while the head voice would be mature, deep and clear with very distinct Major notes in it. Vocalists can use both to give the necessary effects in a song. Sometimes singing acapella can help you refine your head voice. Read more about how to sing acapella here.