I can't tell you where to download free music, that's against the site rules.
You need to create a folder on your SD Card called Ringtones, you can do that using Astro/ES File Explorer on the phone, or in Windows if you mount the phone as a USB Drive. Place the ringtone in the Ringtones folder and it will appear in the list of stuff you have available when you go to select a ringtone.
Make sense?