Your voice comes from your diaphragm, which controls the length and volume of your sound. Expand your midsection as you slowly inhale to properly prepare your diaphragm for singing. If you hold your hand to your stomach, you should be able to feel it expanding as you inhale and deflating as you begin singing. Part of this process is standing up straight and opening up your hips a little to give you the proper room.
A controlled breath is paramount to proper singing technique. You should be aware of your inhalations, and know when you need them the most in any given piece. You need to take longer breaths before longer or more forceful notes, of course. Study the sheet music to know when these breaths should be taken. Hold your body firmly while opening up your throat as you inhale, keeping your shoulders down and expanding your ribs outward.
It's very important before you sing to perform warm up exercises in order to keep your vocal chords limber. The Vocalist recommends an exercise where you take a deep breath all the way down to your lungs, and then let it out in a slow and constant stream. Exhale slowly, he says, and "it may help to picture a candle out in front of you, and your breath is moving so slowly that the flame doesn't flicker as you exhale." Do this ten times, then five more, but with these five breaths, also emit an easy note, holding it constant as you exhale.