Albeit there are many various little correctable factors when learning how to sing, but there are two main things that separate the amateurs from the pros.
Pitch and Resonance. If you have Pitch covered, but can’t seem to get past sounding like an amateur, then resonance is the next hurdle.
You see, there are two general mouth, tongue, and throat positions, speaking position and singing position. Barring a few adolescent pop/rock genres, singing in a speaking position should be avoided. Truth be told, singing in the speaking position pretty much screams ‘amateur’.
Speaking position is pretty much self explanatory. But here is a way you can see it for yourself.
Open you mouth as if you were going to say the word ‘and’ and freeze. As you can see your jaw is slightly open and your tongue is forward in the mouth. This creates a situation where the sound resonance is happening mostly in the nose and slightly in the mouth and none really in the throat, sans the initial sound production in the vocal cords.
Singing in this position sounds nasally and, well, like an amateur. Most people who learn how to sing start out singing like this because it is the most natural and familiar for them. After all, they’ve been using the speaking position all their lives. It’s second nature. It’s automatic.
The singing position, in contrast, is with the jaw lowered a bit more and the tongue further back in the throat with facilitates more resonance in the in mouth and throat and less in the nose.
To demonstrate this, say the word ‘on’ and freeze. That is the vowel portion of the word ‘and’ if sung in the singing position. Go back and get the ‘and’ position, or speaking position, and compare the two. As a matter fact, go back and forth between ‘and’ and ‘on’ until you really get an idea of the difference.
Now there is not need to over exaggerate the singing position of course., unless of course you are going to sing opera. Simply sing with you jaw a little lower and you tongue a little more back and down. You will see a big difference in the quality.
You can read more on this and other important topics in the learn how to sing book, Singing From Scratch.