Learning to sing in your two different voices (head voice vs. chest voice) is essential to total vocal control.
These are essentially two different kinds of vocal tone that you can produce through your vocal glands or your throat.
The first "voice" you have is the chest voice. This vocal type comes from below your vocal chords. This is because we take in air and breathe out from the chest (i.e. lungs).
Your second "voice" is a more compressed form of the chest voice because the air - although it does come from the chest or lungs - seems to be resonating in the area above your vocal chords.
This area is the upper part of your throat and the area in and around your mouth.
You will understand how this works if you pay attention to how your mouth moves 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 tone.
Now, how to sing in your head voice with this mouth position 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…” - go up an down scales, progressively moving higher/lower in key. Make sure you keep your mouth position the as consistently like a fake yawn as possible.
You may also notice that the sound of the notes is almost vibrating in your head.
You will get the hang of it after a few months of practicing.
If you don't want to spend the months practicing and straining to know if you're doing it right there's really only one solution.
Professional vocal training. Now, this doesn't necessarily mean hiring an expensive vocal coach.
In fact, it's much more efficient and effective to learn the fundamentals of singing through an online vocal lesson.
There are lots of great ones, but there are specific lessons on mixing your head voice and chest voice in a program called Singing Success.
I can't stress enough how important it is to do the vocal exercises and lessons these online singing lessons have. You won't find them for free online, but they are absolutely worth it.
If you want to be able to sing in your head voice and chest voice properly in a matter of weeks, not months, you need to pick up one of these programs.
Some vocalists confuse the head voice with the falsetto or the false-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.