The first thing to do in life if a head start on singing is to join a chorus class in school. The younger you start in life, the easier it makes it to adapt to singing later. Don't worry, a voice can be improved later on life.
If starting later in life, it helps to have an singing instructor or a good voice training program--such as one that contains learning CDs. Also, there are plenty of sites on line that will give free singing tips.
Sing along with your favorite songs often. If you can memorize it, sing along with it. Try to pick more challenging singers if the standard ones become too easy--such as a singer with excellent range like John Schlitt or Michael Sweet. Try to imitate the way they sing and make their voices sound. You want to be able to try to stretch your voice as far as they can.
When learning new songs such as hymns that do not have a soundtrack to follow, try to implement some of the voices you sang along with until one is found that goes with the music. Don't try to sing operatically to a song that isn't meant for it. Don't sing high to a song that is meant for low tones. Etc.
Using the above steps, one should be able to improve their singing voice. Don't feel bad if you can't do everything in this article as some people just have different skills than others. However, you might be able to be the next Johnny Cash if you can at least sing some notes--just pick songs you can sing to and suited to your voice. Always remember part of singing isn't just standing there on stage--there is such a thing as showmanship, which makes it into true entertainment.