You will benefit from diaphragm exercises. All your control comes from the diaphragm. When you breathe in, your stomach should go out, not your chest. You have to train those muscles.
Your mouth should be well open. Your index and middle finger, side by side should fit between your teeth, then your mouth is open enough.
Really, some private voice lessons would be in order, at least to learn the basics and learn the exercises you need (though you may find some on Youtube)... then work, practice, work, work, work... just like anything else.. if you want to be good at it.
For finding a local teacher check with the local high school choir director. If there is one around they will know.
Julien