Stand up straight. Fill your lungs so deeply that you push down your diaphragm and push out your ribs. Keep your ribs out, diaphragm taut and shoulders down while letting out the air in a slow hiss. Practice this exercise every day before singing to build up lung capacity and strengthen your diaphragm.
Identify your vocal range with the help of a piano, keyboard or guitar. Play and then sing middle C -- the center white key on a piano. Sing downwards note by note to the lowest note you can reach and hold. Then work up the scale from C. Keep up the breath support and deliberately relax your throat and larynx. Most singers feel comfortable within a range of 16 to 20 notes, starting at G below middle C for sopranos and mezzo-sopranos; C below middle C for altos and tenors; the next A down for baritones; and the next F down for basses.
Note how it sounds and feels when you sing a full, resonant tone at various pitches. To help identify resonance, try visualizing air spinning in the resonating chambers of your throat, mouth and nose. Practice until you can hit this tone every time.
Sing scales, with the piano as guide, to help train yourself to sing in tune. Keep up a continuous flow of breath by controlling it from your diaphragm. To sing high notes, use your diaphragm to push air up into your head. Keep your throat open. If you squeeze the breath through your larynx, it will sound forced.
Listen for tones that sound breathy, nasal, throaty or strangled. To overcome breathiness, build up deep, controlled breathing and practice shaping vowel sounds more clearly. The other problems are due to restricting the spaces available for breath to resonate, so try to keep your throat open and relaxed and don't push back your tongue.