Take lessons. Find a vocal coach or qualified voice teacher who can help you become a better singer. Check with your local music store or your school music teacher for trustworthy references.
If you're serious about becoming a singer, taking lessons sooner rather than later is the way to go: Poor performance techniques can ruin your singing voice for good!
If you cannot afford a teacher, or do not want the dedication that comes with hiring a professional voice coach, consider joining a local choir.
There are also a few at-home vocal training courses out there, such as Singing Success, Sing and See, Singorama, Singing for the Stars and Vocal Release, but be sure to do your research to see which ones have really worked for other singers.