Buy a cheap (or expensive if you're not on a budget) Mic, plug it into your guitar amp, plug your headphones in to your guitar amp, and put some music on your home stereo. The headphones will kinda dampen the treblly sounds from the song in the home stereo and you won't be able to hear the vocalist so well but u'll get the beat and then just sing the song along through the mic and hear it through your headphones, eventually you either get used to your voice or your voice gets better, not sure which one it is in my case :).