Stand up straight. This step is so important, yet so often neglected. You simply will not have adequate breath support if your lungs collapse in because you are slumping. Keep your shoulders over hips, shoulders pressed down, feet comfortably apart, chest open, chin lifted just slightly. This will maximize how open your airway is.
Start with some breathing exercises. Breathe in and hiss air out; hum on different pitches; go up and down a few of the easy major scales on a staccato "Ha" type noise to get your diaphragm ready to go.
Pick an easy note in the middle of your vocal range. Middle C is a good option for altos; the G above for sopranos; the G below for tenors; the C an octave below for basses.
Sing that note at a mezzo piano (mp) level on a syllable such as "Ah" and hold it for as long as you can at that volume.
Repeat Step 4, but this time start at mezzo piano (mp) and go through piano (p) all the way down to pianissimo (pp), which should be the quietest and softest you can possibly sing without straining.
Begin at mezzo forte (mf), using the same note and syllable as you did in Step 5. Increase the volume of this tone to forte (f), all the way up to fortissimo (ff). This should be the loudest you can sing with good breath support and without straining.
Commence singing at pianissimo (pp), using the same note and syllable as you did in Step 5. Work your way all the way up to fortissimo (ff), remembering to keep your pitch and syllable constant and change only the volume at which you are singing.
Reverse what you did in Step 7. Begin at fortissimo (ff) and work your way all the way down to pianissimo (pp). Again, the pitch and syllable should not change, only the volume.