Proper posture is essential for your breath to be supported correctly. Stand with your feet shoulder-width apart and try not to sing while sitting. Make sure your shoulders are back (but not rigid), and the muscles in your bottom are tight. Keep your abdomen muscles loose, and no matter how self-conscious you are, do not suck in as this will tighten the air flow. If you must sit while singing, do not cross your legs as this can throw your body off center.
Although a widely debated topic as to how its done, it is universally agreed upon that breath support is of utmost importance. Proper breathing in singing consists of gaining air from your stomach and not from your chest. Try taking several deep breaths with your diaphragm (located in your stomach area, underneath your ribs) while standing in the proper posture. You should work on feeling no tightness in your stomach as you exhale the sound out of your mouth. It may feel funny at first, but it will improve the quality of your voice over time.
Harold Hill says, in the Broadway classic "The Music Man," "Singing is just sustained talking." His character is right, to an extent, and the theory is expounded upon in "The Actor's Survival Kit" by Miriam Newhouse and is echoed by "The American Film Musical" by Rick Altman. How you speak can very well help or hurt your singing voice. Talking in the back of your throat can weaken your vocal chords, so it is important that you learn to speak correctly. Speaking correctly is just a form of using better breath support, so when you speak, you are supporting your words just like you are with singing. Many singers also take acting classes or speech therapy to improve their talking voice so they are not inadvertently damaging it during non musical activities.
It is also important to stay away from caffeine, dairy and alcohol as these can coat the vocal chords (in the instance of dairy) or dry them out (caffeine and alcohol). Clearing your throat is another habit you're going to want to rid yourself of. This injures the vocal chords by slamming them together, making your throat hurt and feel scratchy. If you have phlegm in your throat, you can hum to get rid of it or drink a glass of water.
Do not scream. This is important. Screaming can potentially be more harmful than clearing your throat, so it's best just not to do it.
Pushing your voice is a common problem with amateur singers. Do not push your voice to reach a note you cannot reach naturally. This can result in damage to the vocal chords. Instead, relax and practice. In time, you may be able to reach the note in a healthy way.
Practice singing as much as possible using proper posture and breathing. Try to get recordings of songs or the scales without vocal accompaniment. Wareseeker.com has a fabulous resource of free or cheap scales that can be procured online (see Resources), although you can also purchase them in your local music store. You can also record yourself singing on a tape player or on your computer and play back your practice sessions. This way, you will get an idea of what you need to improve on without teacher assistance.