Find your diaphragm. This can be done by pressing your hands firmly into your sides, and breathing in deeply and slowly. Feel your sides, stomach and back expand. This is where the air needs to go when you sing. If your shoulders move and your chest puffs up, you're inhaling incorrectly. Take the air into your stomach. You'll need to inhale much faster when you actually sing, but this exercise teaches you how to move the proper muscles and where to store the air. Too often, singers store the air in their upper lungs, which results in a breathy tone.
Place your hand on your diaphragm (the area just below your rib cage) before taking a deep breath. You should feel the area around your stomach and sides fill with air. Let out an "Ahhh" sound out as slowly as possible. Feel your stomach contracting as you let out your "Ahhh" sound. The object of this exercise is to get your stomach to contract as slowly as possible. This is teaching you breath control. If your singing is too breathy, your stomach will contract quickly and air will be let out from your lungs.
Practice the lip roll exercise. To do this, keep your lips together and breathe out through your mouth. The resulting sound should resemble something similar to a horse or a race car. If you're having trouble with this exercise, try it while pressing on your cheeks with your index fingers. This will support some of the extra skin while you breathe out.
Practice crescendoing and decrescendoing on a single note. Crescendoing is the process of getting louder, while descrescendoing is the process of getting quieter. Holding out one long note while getting louder and then quieter will teach you breath control.
Practice solfege (do-re-me-fa-sol-la-ti-do) scales in single breaths. Practice until you're able to get two or three scales down in a single breath. This will teach you to hit various notes while controlling your breath, making practical use of your diaphragm and breath control exercises.