Warming up is essential to avoid straining your voice. There are a number of exercises that will help, using the familiar "do-re-mi-fa-so-la-ti-do." "Do-re-mi-fa-so-la-ti-do" is the vocal way of singing a complete scale. If we pick middle C on a piano as "do", the complete scale will be "c, d, e, f, g, a, b and high c". "Do-re-mi-fa-so-la-ti-do" is sung instead of the note names as the sounds are easier to sing than the note names and simpler to remember.
The simplest scales exercise is to pick a comfortable note to use as low "do" and sing a complete scale ending on the upper "do." When you have completed this scale, start it again on the "re" of the first scale and again sing the entire scale. Keep singing these scales, each one pitch higher than the last for as long as you comfortably can.
For variety, try a "do-me-so-do" scale and repeat as above. The "do-me-so-do" scale trains your voice to leap notes. If we use middle C on the piano as our low "do" again, the sequence of notes is "c-e-g-high c".
Try humming these scales first and singing after; this will ease the voice into opening up.
Be sure your posture is correct before you sing as incorrect posture can affect your air flow and restrict your breathing. Stand with your feet evenly spaced out on the floor, your knees loose and your body relaxed. Keep your bottom in and your back straight. Keep your head up; imagine there is a string pulling it up straight.
Breathing exercises are a must for any singer. Proper breathing will help you to hold notes longer and to keep on key.
When you sing, your shoulders should not move. If they do, you are not breathing from your diaphragm (the muscle underneath the lungs). To use it in singing, be sure you are breathing properly. You can do this by making sure that when you are breathing normally, that you are pulling air in through your nose and out through your mouth. As you breathe in, your stomach should move out and not in. When you breathe in, your stomach should pull in. Hold your hand on your stomach and practice until you are breathing correctly.
When you speak, you do not tend to open your mouth very far to release the sound. When singing, you should make sure that your mouth opens wide to release larger notes. Even when you are not singing large notes, the mouth should stretch to exaggerate each syllable. Practice stretching your mouth by saying the vowels, widening and exaggerating your mouth as you do.
Good pitch and voice tone comes from using your voice correctly. When you speak, you tend to speak from your throat. When you sing, you should not use your voice in the same way. When you sing, you should imagine that your voice is travelling up through your head.
Your diaphragm should control the note if you are breathing correctly. This does not mean that you should sing through your nose-- rather, the roof of mouth should house each note. Your bottom lip and teeth should move more when you sing than when you talk.