Pushing yourself to the limit of your speed and endurance over a sprinting distance isn't an easy task, and holding your breath isn't going to help -- that's for the people watching to do as they wait for you to win the race. Your muscles need oxygen to function at full capacity, and breathing properly while sprinting is essential.
When you're in position and ready to run, inhale and hold your breath until you get the signal to go. Exhaling powerfully as you push off helps increase your momentum and center your mind so you can focus on the sprint. After exhaling, it's fine to run several steps without inhaling as you find your rhythm, but start breathing before you begin to feel out of breath.
Use Your Mouth
Mouth breathing might not be attractive in a potential mate, but as a sprinter it's the best way to get air into your lungs. Opening your mouth slightly relaxes your facial muscles, and you'll be able to concentrate on pumping those legs and arms during the sprint. It also lets you bring in air through your mouth and nose simultaneously for the highest possible amount of oxygen per breath.
Breathing with Your Belly
When practicing your sprints, place a hand on your stomach and your chest, and pay attention to which hand is moving when you breathe. Your chest hand should stay mostly still, while your stomach hand should be moving with each breath. This means you're engaging your diaphragm, which increases the space around your lungs as you breathe in so you can take deeper breaths. With chest breathing, you're confined to a smaller chest cavity. If your shoulders are going up and down as you breathe rather than breathing with your belly, you're wasting energy that's better used pushing your legs to run faster for the short dash.
Sprinters pop into stride quickly -- they don't have much time to find a rhythm before the race is over. Cadence breathing is common for long-distance runners, but it's helpful when sprinting as well. Keep your breathing -- and your nerves -- under control by timing breaths with your steps. Inhale for two steps and exhale for two steps, for example. For longer sprints, inhaling and exhaling for three steps instead of two might help keep your stamina up for the race's duration.