Breathe from your diaphragm. The diaphragm is an involuntary muscle that you have some conscious control over. Without this control you wouldn't be able to manage your air flow. When breathing, expand your sides and stomach and fill your lungs with air.
Sing a five-note scale starting on middle C for women, and F below middle C for men. Play your starting note with your tuner. Match the pitch and then ascend five half steps. Watch the needle on the tuner and make sure to sing each note in tune. Descend back down in half steps to the original pitch. Start a half step higher and continue the series until you have hit the top of your range.
Vocalize the sound "hmm" while starting on a pitch that is approximately a fifth lower that the highest pitch you can sing comfortably. Sing down through half steps a perfect fifth, which is a total of seven half steps. Start again a half step higher and sing the same series again. This exercise will help improve your vocal flexibility.
Sing major scales using whole notes. If you do not know how to sing a major scale ask a pianist to record himself playing all 12 major scales in two octaves. You can also use a tuner to provide your first pitch and then sing the notes: C through B without accidentals or sharps. You do not need to sing scales in every key, the important thing is to develop your tone by holding out long tones.