Singing is easy, have that in mind. If you compare singing to playing an instrument, you'll know what I'm talking about. Why? Because you already speak, and singing isn't that far from it. You modulate your voice and mouth to produce the sound you want in speech - it's the same for singing.
Learning to sing is much more like learning to play asport or dance - because, as I said previously, your vocal tract is a muscle, and as so requires training and coordiation.
As Phobia said, simply singing daily can greatly improve your singing. Why? It's just like learnign a language or an instrument on your on. Harder in the beginning, but you soon make drastic improvements.
Warmups
First thing of all: warmups. Take it like this: you wouldn't be able to run 4km without warming up first. The same goes for your muscles. Singing without warming up is not only unhealthy, but it will greatly hinder your singing potential.
Singing itself may help to warm up yoru voice, but it will take longer and it could harm your voice if you're not careful.. so an beginning exercise is recommended, as explained below:
So what to do to warm up? First you should stretch your voice by choosing a note and singing it for as long as you can keep it. This will prepare you for using your full vocal range and potential. After doing this a few times, you must do a progression. A progression is somethign like: lah lah lah lah lah lah lah. Doing it a few times will help you to smoothly delve into lower or higher notes as you sing.
If, after the warmups, you still ruin some basic notes, it may be necessary to clean yoru throat, but in a way not to hurt it. For this, some fresh lemonade - lemon helps to clean your throat :D
After you feel nothing is holding your voice aback, you may start to sing.
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Breath
Singing is breath. All singing techniques involve breath in some way. Therefore, having the ability to breath in deeply and letting it out proficiently is the base of any singing technique. Some people ruin their singing by breathing too hard, making their voice sound panty; and some others ruin their singing by breathing too softly, therefore making their voice break easily (and it can also harm your voice).
As a rule, singing higher notes requires more breath. But don't exagerate :P
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Singing styles
Now for the techniques. You must reckon there are different techniques you use for singing in different rangelengths. According to how high notes go, they classify as the following:
chest voice
head voice
falsetto
high falsetto
The chest voice is the lowest of the vocal techniques. It is so called because your voice echoes in your chest before coming out. You may recognise you are singing in this register by touching your midchest with your hand and seeing if it vibrates. The chest voice is a very steady and trustworthy register: it's not gonna fail if you try to sing louder.
The next one is the head voice, which is higher than the chest register. It is so called because your voice echoes in your mouth and nasal cavities before coming out. Akin to the chest voice, you may recognise the head voice by feeling your head vibrate as you sing using it - more specifically, your nose and face. This is usually the register with the widest range, although it may be different for some people.
There is also a mixed style of chest voice and head voice. It is the style most people speak in, and is very useful because you can progress into chest or head voice from it with ease. As expected, you should feel both your head and chest vibrate as you sing this way.
The falsetto register is most known among guys, since they use it to produce higher notes than the usual masculine range. It is called falsetto, which is italian for "false voice". It is called that way because, when singing falsetto, you reduce the length of the vocal chords you use to produce a sound. Physics lesson #1: a shorter chord will vibrate in a higher frequency.
There usually is a gap between teh falsetto voice and the head voice, which makes people able sing either too high or too low, and not be able to make a smooth progression. In fact, some singers spend their whole lives dedicating to a in-tune progression between head voice and falsetto.
With falsetto, you will usually stop feeling your face vibrate, although it may still happen. Furthermore, some people cannot sing in falsetto - because they cannot use it to produce a sound which allows for speech (as if you were making a parody of singing, as you said). But there are ways to fix this. A main way of doing so is by emphasizing vowels - making them "open".
A rarer singing register is the high falsetto, most known by the name whistle register. It bases on the same idea of the falsetto - using less of your vocal chords to produce a sound - but in a completely different technique. It is called whistle register because... heh, it is so high that it resembles a whistle. Singing lyrics in this register is impossible though - so it is only used for screams and etc.
If you want any help on a particular singing style, just shoot.
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Vocal registers
Another thing to have in mind is the different singing register. Singers' vocal range is classified according to a scale of pitch. The are basically the following:
Male voices:
bass (the lowest)
baritone
tenor (the highest among the average male)
Female voices:
alto
mezzo-soprano
soprano (the highest)
In childhood, both male and female voices are the same. However, when boys reach puberty, hormonal changes affects their voice, making it the low, manly type. It is not rare to find a young boy soprano in choirs.
But keep in mind there are no restrictions concerning register. Many men can sing well past the tenor range, sometimes reaching a full-blown soprano. To do so, they usually use the falsetto technique. These singers are called countertenors, and their range can greatly vary.
The same applies for women - some can sing much lower than the alto range, but some quite higher than the soprano one. It really varies from person to person.
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Vibrato
Another thing that is very important to your singing is the vibrato. You know when those singers sing like aAaAaAaAaAaAaAaAa? As if their voice "vibrated". Apart from making your voice more beautiful, it may also help you to sustain higher notes.
To do a vibrato, you must use your diaphragm to vibrate. A vibrato is pretty much about controling your breath and making your breath vibrate, instead of your voice. My hint is just to keep trying and do it: in the beginning, you may find you'll need to aid it with your voice, but after some good practice you'll be able to do it smoothly. Another hint it to try to pratise the vibrato when only breathing out - without using your voice.
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Reproducing a note
The final aspect of singing is the ability to reproduce a note. Some people screw their singing by getting too excited and singing a note higher than it would be, or by fearing that they will not be able to reproduce a specific high note and therefore making others lower to make up for it. That's why singing songs that are within yoru range is important - although singing songs that are slightly out of your reach can greatly help you to expand your range.
For getting the right notes, my hint is - practice, practice, practice. You'll only be able to sing a song beautifully if you've trained it.
Finally, hearing your own recorded voice is very important to know what needs fixing in your voice - that's because you don't hear your voice the way other people do. Sometimes, a note may sound so beautiful to you, but is actually a piece of shit to others. Having people's feedback is always a good option.
So I guess that's it. Anything else, just shoot :P
PS: sorry for typing mistakes
