Garage Band & pre-recorded music

I am trying to change the key to a pre-recorded Karaoke song. I've been all over the internet and have found all kinds of advice. It's rather overwhelming! Please, someone tell me that I can do this in Garage Band and HOW. I've read the manual and watched several tutorials but not once do they mention changing keys to a pre-recorded song. My Dad (82) wants me to record me singing to one of his favorite Karaoke songs. Unfortunately the key is too high. I need to lower it. Any help would be greatly appreciated! Thanks much. L.A.