I don't believe there are any real spells for that. But in all seriousness, you should practice daily. If you have the resource you can get a tutor for singing and playing the guitar. Just practice and don't give up even when it seems too difficult to continue.
jackal's right. pray to a deity of music for help, but you still need to practice. all spells are like this there's no overnight fix, you need to be productive, even a little bit. pray to a musical deity and practice.