I have found iTunes.com to be the best and most dependable for music downloads. great quality, great selection - easy to use. I can edit the song length or the volume. Just download the application for free on to your desktop; set up an account, and buy music. Ask friends/family members to give you iTunes gift cards (found at Walmart, grocery stores, online) for your bdays or Christmas. And all the CDs you own now can be uploaded into your library so if anything happens to the CD (scratched, stolen, lost, you always have a copy of it on your desktop). I've made a playlist of children's songs for my son and it's great... iTunes has a whole genre section of Children's music.
I'd be leary of any "free" file sharing website -- remember what happened to Napster customers? [The big record labels went after regular Joe customers, sued them in court for stealing music and won!] If you're buying rights to own the music, you need to pay the artist something for his/her creation. Stealing music is not right to the artist.