I can't teach anyone much about guitar, but there's one thing I know very well, and that's singing. I have been singing forever, and so I'm going to impart that knowledge to the masses of guitar.com. Because you might want your songs to have lyrics. ;o) Here's some tips for opening your range and improving your projection:
 
Start off by clearing out your throat. No dairy products for at least an hour, no soda, no smoking. Drink a highly acidic beverage, such as lemonade or tea with lots of lemon. The acids in lemons help to break up the mucus on you throat, because mucus makes the notes sound bogged down.
 
Get in a few good deep breaths. No crazy "memememe's" here. Just get a good deep breathing pattern going to open up the diaphram.
 
Sitting as straight as possible, just start belting out you favorite song. It helps to use music you like because you can really put the passion into it. I personally use Queen's "Bohemian Rhapsody"...it's fun and has a great tonal range.
 
Do this process at least three times a week for a few weeks, just to open up the vocal chords and get you comfortable in your own voice. Then, start singing without the music playing. Test yourself. See how high or low you can go without cracking. For the higher notes, bring your chin into your chest. That allows the larynx to move freely and achieve higher notes. For lower notes, tilt your chin upward to open the throat and give the larynx room for deeper vibrations. As you do these exercises, you will be able to hit higher and lower notes, and you will increase your projection, which is great when you are trying to sing and play guitar at the same time; the microphone isn't always close to your face. Try recording yourself and listening to the playback to see what you can do better.
 
You don't have to be a natural born singer to sing well. A lot of skill comes from technique, which all guitar player know. Now start making some great music!