This is pretty idiot proof with arrows and all that jazz...I shouldn't even have to type out these steps but here we go...
#1 Go here...
#2 Click here to go to youtubes' video editor
#3 Make sure you have ALREADY uploaded the video you want to add music to (IF YOU HAVE NOT, upload first then come back here. Now select your boring video (boring cause it has NO MUSIC), and drop in the timeline
#4 Click that music note, type in some keywords as to what you are looking for. This might need some playin' around before you find what you really want. Again drop it in the timeline, but the AUDIO ONE.
#5 save your video (do you really need an image for that?
Thats it.