I've been singing since I was a very small child, my dad was a musician, and I remember watching singers on television for their mouth and vocal tension in holding out notes. There is a lot of mouth control involved and I observed this from the age of five or six.
HOWEVER, what is even more important than that is singing from your soul, or what most people technically call the diaphragm. Natural inclination and years of choral training taught me that the most important thing you can do is take deep breaths and project from deep inside yourself, it makes your voice stronger and richer, and less likely to crack.
No one can give you an "ear" for music, but you can learn to sing from the place where your lungs and your tummy connect. It has to come from deep down within yourself, both physically and emotionally. Confidence helps in this matter. If not confident, loud showers and isolated vehicles can be used as practice zones. I also suggest the woods, and when you're home alone.
It feels great to take deep breaths and sing from far down inside of yourself. It has to come from there.