Use mixcraft. You can download the (14 day or somrthing) trial free, and it syncs up good with a nice mic. Just load the instrumental into mixcraft as a track, and hit record on a new track to do your vocals. After the trial's up, you can use audacity to play whatever your finished rap is (and run audacity to record the audio, then export that as an mp3/wav). Or if you don't wanna go through the audacity trouble when the trials up, use craagle or something and find a serial code for the full version. There's alot of programs for recording audio out there, but i've always liked the craft best.