From Science Daily, reporting on research from Northwestern University:
The ability to sing on key may have more in common with the kind of practice that goes into playing an instrument than people realize, said lead researcher Steven Demorest, a professor of music education at Northwestern’s Bienen School of Music.
“No one expects a beginner on violin to sound good right away, it takes practice, but everyone is supposed to be able to sing,” Demorest said. “When people are unsuccessful they take it very personally, but we think if you sing more, you’ll get better.”
Singing well requires practice. And don’t ever let anyone tell you that you’re tone deaf. Just keep singing.