Professional singers use their diaphragm. It is located just above the stomach, below the ribs, and between the lungs. This is what you hit when you use the Heimlich maneuver when someone is choking. Exercising this can help you sing higher, lower, longer, louder, and enable you to take in more breath quicker.
First, you have to retrain your breathing. Most people are actually breathing wrong, and breathing correctly can help you lose fat and gain muscles quicker. After you exhale, inhale to the fullest capacity your lungs can hold for a steady 4 beats. Then hold it for 4 beats, keeping the same pace. Exhale for 4 beats, and hold for 4 beats. And repeat. You can tell that you are not using all of your lung's capacity if you are filled up before the 4 inhaling beats are done. Learning how to breathe correctly may be hard, but with practice has some very noticeable and satisfying results.
If you do this one exercise at least 20 times a day, you'll exercise muscles we rarely use anymore. It will be very hard at first! But you may even gain abs! Lay down flat and, using the correct breathing technique stated above, slowly lift your feet 6 inches above the ground while you inhale, hold, and slowly move your feet back to the floor while you exhale, and repeat.
Good luck!
Edit: Voice lessons have helped me tremendously. However expensive, it is good to put out a little money if you hope to have singing as your career. Agents/producers look for someone, not only with talent, but with experience and training. They don't want someone who may hurt their voice soon after getting popular, thus not making any money. Don't learn the hard way!