iPhone question about ringtones?

Got the iPhone 3gS today. I trimmed a song down to 30sec but it wont allow me to put it as a ringtone because it wasnt purchased from itunes. So when i did purchase the song, and right clicked on create ringtone, it keeps coming up as an unknown error and to check my network connection. Any help would be appreciated! cant use ventones, i am in the UK and it wont register a phone number for me