You could go onto zamzar.com.
I use it all the time. You find the song on YouTube, put in the url and put it into mp3 format. It'll go straight to ur email, you save it into ur computor, and when you go onto ur iTunes you go File, add to library and ur done!
also, try to get songs off of YouTube with good quality so you get good quality and make sure the song cuts off right when it's suppose to so you don't get 15 seconds of nothing at the end.