When you sing, you use your diaphragm to push a lot of air through your mouth. This gives your voice power, tone, and volume. Some singers will be able to use their diaphragm properly, but some of the air will spill over into their noses. If you want to learn how to stop singing through nose, you need to learn how to open up your mouth. The best way to do this is by lowering your soft palette. Take a good yawn, and feel the way that your tongue moves. This is the position you want the back of your tongue to be in when you sing.