Some of us have sweet clear voices. Some people have raspy voices, edgy voices, soothing voices, emotional voices. Some people have a certain way of vocalizing words. We're all different in our own glorious way.
If you have a high sweet Mariah Carey voice, but you want to sing Melissa Etheridge songs, be careful! You may end up thrashing on a weak spot in your voice and hurting yourself. Which is walking backwards. You may increase your range or broaden your style, but you should proceed with care and awareness.
Start finding your own singing voice by listening to your talking voice. Get a tape recorder and tape yourself in a casual conversation, play it back and listen for the pitch and timbre you're talking in. Find a complimentary key on an instrument, and start singing in and around that key. Now you have found the beginning of your natural comfort zone in your voice. This is the most important part to work on first. Then you can stretch your voice "slowly".
If you play an instrument, then find songs in a key close to the comfort zone. If you don't play an instrument then play CD's and sing along with songs that feel good to you. A great way of stretching your vocal cords slowly is with "Ma Me Ma Mo Moo" scales or "Doe Rae Me Fa So La Tee Doe" scales. Starting as low as you can, sing to as high as you can sing. If your throat begins to hurt, stop for a while and try it again later. No good comes from hurting your throat, it just makes it harder to sing later, and your range becomes smaller.
Of course, the more you sing correctly, the better you get. It's just like the other instruments (but more sensitive). It can take years to master. So don't give up. Be proud of what you accomplish today. Work on what you can learn tomorrow.
The Singer In The Band