how can i record a song in a karaoke music track via GB?

First plug in some headphones.

Then drop the karaoke song into garageband. You do this by just draging and dropping the song file into garageband or if the song is in itunes just use garagebands media browser.

Now create another track (vocal track).

Put headphones on hit record and sing.

This is a bit over simplifed but give it a try someone will be here when you have more specific problems.

How would I do this on a MacBook Pro with only one input? Should I get some usb headphones if I have a mic that will plug into the audio in/out jack? Or should I buy a splitter of some type? I want to be able to listen to the music on headphones while I'm recording into a decent mic.

Thanks so much for your help!

A splitter won't do because the combined in/out jack can only do one thing at a time.

Plugging a mic into the line-in won't give you a good signal anyway. You need to pre-amp it somehow, but then you can't monitor with headphones. A USB headset is one option, as you said, but I would probably go the other way and get a decent USB mic and use the jack for the headphones.

Your question is a little confusing. Do you have a karaoke song already and want to add a vocal or instrument track to it?