Song Editing Question, Help PLEASE! it's VERY important?

So I'm seventeen and not really what you'd call a computer wiz. I figure things out as I go, but a friend asked me to edit two songs together for her wedding which is tomorrow because I managed to figure something out once! I'm using Audacity so I was wondering if there is a way to either sererate the voice from the music or if there is ANY advice for a better quality mix than what i ended up with! PLEASE PLEASE PLEASE HeLP!!