Conceptualize clearly the pitches and rhythms that you need to sing. In order to sing clearly and confidently, you must first have a clear idea in your head of what you wish to sound like. Taking the time to vividly imagine all of the pitch progressions, before singing, will ensure that you can sing your part, without being dependent on hearing it sung by someone else around you.
Breathe deeply, expanding your diaphragm first from your stomach, then moving up into the chest, and finally expanding outwards. The breath is where all of the energy of singing comes from, the vocal cords merely channel and focus it; so proper breathing is the most important part of any singing.
Relax your vocal cords as you begin to channel the air from your lungs through the vocal cords. Low frequencies vibrate at slow frequency, so a more relaxed vibrating medium is required for resonant sound, and for vocal health.
Feel that the air is flowing freely through the vocal cords, activating them as it effortlessly passes through. Singing in as relaxed a manner as possible creates the best sound, and puts the least amount of strain on the vocal cords; so it is important to feel as if the air is naturally flowing outward, rather than that you are deliberately pushing out air.
Enunciate clearly and powerfully the consonants in every word. Particularly in large performance halls, the consonant sounds get muffled, so popping them out with more force than usual will allow for clarity in any setting. This also helps begin singing with clarity and power.
Check yourself often to be sure that your whole body is relaxed and that you are taking deep and nourishing breaths.