You call up a teacher and make an appointment. The teacher usually sits at a piano and plays things for you to imitate or sing back to her. She will test your range and give you exercises to find out if you have breaks and whatever your issues are, and then give you homework. It's like learning and practicing any other instrument. You learn basic technique and you learn technique specific to your voice. The teacher selects songs for you but you may also get to pick things you want to sing. Usually, the more you practice, the better you get.