Do not take posture for granted. When you sing, whether solo or with a group, much of your success comes from proper breathing techniques. You must avoid taking gasping breaths in between pauses when you are singing; therefore, you need to make use of your abdominal muscles. Abdominal breathing requires excellent posture. Relax and take a deep breath. Stand up straight. Point your chin forward and keep your head level. Point your toes forward and distribute your weight evenly on your heels and soles. Confirm that your posture is correct by placing your hand on your stomach while you sing. You should feel your stomach expanding as it fills with your deep breath.
Learn to breathe. Cradle your belly by placing your palms at the bottom of your rib cage so your fingers splay across your abdomen and touch each other. Take a deep breath and note how your fingertips separate. Now hiss, sustaining it for a full minute while keeping your rib cage and sternum elevated. Inhale and then hiss five times in a staccato. Inhale again and this time hiss twice staccato and once sustained. Inhale and sing a syllable such as "saaah" in a midrange voice. Proper breathing will help you eventually hold notes longer without running out of air or voice.
A good singer will not produce a nasal voice, but a balanced voice will include some nasality in the sound. To see whether you are striking the proper balance, sing something while pinching your nostrils. If the sound of your singing does not change as a result of you pinching your nostrils, then your singing is not nasal. If it changes very slightly, then it has just the right amount of nasality. If the sound does change, then your singing is much too nasal. Place the tip of your tongue behind your upper teeth and hum, staying midrange, while you press your nostrils and release them. Do this until you reach the proper balance of nasality in your voice.
Practice the scales often. Doing so not only helps you work on diction and articulation, but also on voice extension. Sing the arpeggio "do-mi-sol-do-ti-sol-fa-re-so" and "do re me fa so la te do." Start doing it on different keys to test how low and high you can get your pitch to go.