You came to the right place ; ). The most important thing about singing is breathing. When you breathe in your stomach should go out and should stay out as you sing.
To practice this lay down on the ground and place a phone book on your lower stomach(diaphragm area.) If you Breathe properly the book should fall off every time.
Some people have big mouths so there constants are too wide. The best practice for this is to watch yourself in the mirror and but two finger on each side of your mouth. Sing and make sure your mouth doesn't go past your fingers.
That's a good starter lesson if you want more you should really consider voice lessons. Yes they are expense but they're worth it.
P.S. Drinking gallons of water each day helps your voice tremendously. (and some tea in between). Good Luck.