There are loads of different way. If you have realplayer it will download it for you, but probably the easiest option is to DL youtube downloader from here:
http://youtubedownload.altervista.org/
This will do it for you, also, if you DL a music video this will give u the option to then change it to an MP3 for your ipod or whatever. My kids use this to get the music they want.
If you have any difficulty with it PM me and I will guide you through it, but it is pretty straightforward.
John