Singing takes muscles. The only difficulty: The muscles used in singing can't be seen easily. Singing occurs when air flows through the larynx (voice box). Within the larynx are folds of neuromuscular tissue, the vocal cords. These folds tense or loosen, causing sound-producing vibrations. A singer relies on the strength and flexibility of these small “muscles.”
The vocal cords are delicate, and the first step to training your voice is caring for them. They must stay well lubricated, so drink plenty of water and stay away from dehydrating beverages such as soda, alcohol or coffee. Thomas Murry of University of Pittsburgh Voice Center also suggests staying away from places where you might try to "talk over" noise or "whisper loudly for very long." It is also important to be mucus free. Avoid smoke and other pollutants. If you have a cold, consider medication that will clear out mucus (be wary of medications that would over-dry your sinuses). Herbal tea or the hot steam of a shower can loosen mucus and relubricate dry vocal chords.
You’re now hydrated and mucus free. Do some light to medium stretching: If your body is tight and cold, your vocal cords will be too. Now that you are limber, look into the mirror and make a ‘hisssssss’ sound (like steam out of a pot). Keep a straight face: We are focusing on a steady stream of air and require no face muscles for this exercise. Once you can maintain a stream of air, add a ‘zzzzzz’ sound at a conversational tone. Maintaining the same straight face, stream of air and ‘zzzz’ sound, go up and down in pitch slightly. The vocal cord is like a rubber band, and we are slowly stretching it a little at a time. As you feel your voice loosening, take another breath and stretch your pitch further up and down. Once you feel warm, go through the same process at a louder volume. Continue until you reach a strong and comfortable singing volume.
Now that you are warmed up, you can sing--you're ready to practice songs or scales.
Go through the warm-up process but in reverse. Do your ‘hisssssss’ and ‘zzzzzzz’ at a singing volume and work your way back down to a low conversational volume. The key to strengthening your voice is the time spent warming up and cooling down. All athletes have to work out properly to avoid strains and blowouts. Singers are no exception.