Music must be added to the video BEFORE you upload the video to youtube. That means whatever you upload to youtube is exactly what will be displayed/heard. You can't upload it to youtube and then add the music.
You need some video editing software. I purchased a MacBook Pro for the main reason that it has iMovie and I find iMovie to be a pretty easy video editing software program. There are several free video editing programs available on the internet. It will take a bit of learning to really get creative with the sounds and transitions, but if you are like me, you'll really enjoy it.
You can view some of my videos at Micah Kneidl - YouTube
to get an idea of some of the projects I have put together. Some pretty fun whitewater videos in there if I do say so myself.