Yep, I only sang or hummed to myself in the car. Then I wanted to learn to sing cause I had a uke. I took voice lessons from a Classical guitar/piano/flute/voice instructor. I was able to develop more range, especially in the upper registers. We worked for about an hour, once a week. We usually worked at the piano. She had me look in a mirror to correct mouth shapes, and we did lots of scales. We worked a lot on breath control. She would give me the melody parts and sing harmony, to try to pull me off. I had to stay on melody. We left off just as I was supposed to learn to sing harmony. Maybe I'll go back.
Don't ask how much it cost, she wouldn't take any money. It sure did give me a lot more confidence. Now I actually enjoy singing and have gotten several compliments!
Good luck!