[Spellbound]

Really, the best thing that you can do is practice. Practice a little bit every day (if you practice too much, your voice will get tired and worn out, so don't overdo it).
I used to take voice lessons and it really helped me get the roles that I wanted, because the vocal coach went over the songs with me and corrected my flaws. It made me ten times better of a performer.
If you don't think that you can afford such classes, practicing alone will do you good as well. I recommend practicing in front of the mirror. That way, you can work on your facial expressions, etc... And that will make you more comfortable with your acting.
I hope that I helped. Good luck.