Best answer by Mathis
Hmm, so as far as I understand you're looking for a way to overdub your guitar recordings. If so, you can use any audio editing software that is capable of handling multiple tracks. Audacity is a good way to start, since it is freeware and works on most operating systems (OS X, Windows, etc.). This way you can play back your quitar song on one track, while recording your voice on another track simultaneously.
Hope this helps for starters.