Nasal sound and thin sound are almost always caused by one of the following: 1.
Singing with a pinched or closed back of the throat.
(Say "ga-ga-ga" and you will feel the back of the throat closing and opening.
The hard "g" brings the back of the tongue up, and the top of the throat down so they touch.
The "ah" opens them up again.
) When this opening is too small, the voice sounds thin and muffled.
Think about the way your stereo speaker would sound if you put a board with a small hole in the middle in front of it.
The full sound just couldn't get out.
Imagine how that would sound.
Nasal sound occurs when the back of the tongue rises even just a little.
Test this out by singing an "ah" with a very open throat.
While you are doing this, pinch your nose.
If your throat is open as it should be, the will be no change in the sound.
If the sound gets nasal, your tongue is rising up as you sing.
Try it again until you get a clear, non-nasal sound.
Now sing the clear sound, only this time, just before you pinch your nose, let the back of your tongue rise a little.
The sound should become noticeably nasal.
A good exercise to train your tongue to stay down is to sing a series of "ha-ha-ha"s.
Let the "h" blow your throat open.
This doesn't take a lot of pressure.
Alternate short scales of "ha-ha"s" with straight "ahs".
doing this often should train your tongue to stay down when you sing, instead of reflexively rising.
2.
The second major reason for having a nasal sound could be a deviated septum.
This is a malformation of the nose bone and cartilage which re-routes the sound more into the nasal sinuses.
You could have been born with this, or it may have been caused by a blow to the nose.
Unless your deviated septum is severe (have an exam by a specialist), opening the throat will also help you overcome the nasal sound.