I don't know of any Internet web sites that teach singing, but I would suggest that if you have the time and money, taking a singing class either in a group or with a private voice coach would probably be preferable. I took a semester of voice with a very compatible group of students. If you are singing for a real, live teacher, there is so much more they can tell you about your voice and how to improve it and bring out the best. I don't have any talent for singing, but even so, I believe that class helped me tremendously. A web site just couldn't offer you encouragement or tailor the course especially to your needs. There are books and videos that claim to help improve one's voice also. I bought one such book, but got so much more out of the class I took. Trying to learn to sing from a book or web site is like trying to learn to play golf or tennis from a web site. It just doesn't compare to having a real, live human coach.
i took a site singing class this summer...good stuf...now im taking singing classes durring college...the singing class is well worth it. peace chad