The best you can do is practise.
Magick will not make you get a better singing voice. Magick does not work like this. It won't make you play an instrument better, sing better, draw better, etc. They are merely arts/talents that are developed by practise. To do better in such things, dedicated practise and patience are the keys to success. It is the one and only thing that will work. Magick is spiritual and it has it's limitations. You cannot make magick to work in instance and get you a better voice. Expecting such things is unreasonable.
Join a singing class and practise. With time, I am sure that you will do better. I wish you luck! :)
The reply above me is correct in that there isn't really a spell that would work. I will suggest that in addition to your rigorous practice, you also incorporate meditation. Meditate on how air fills your lungs when you breathe in, on controlling the diaphragm muscules, and on how your vocal cords move when you sing. Being aware of your body helps with many things.