The previous post [removal required by the Verizon Wireless Terms of Service] is incorrect.
I love ringtones myself Duanebrenda! If you downloaded the song as a song and not a ringtone then it will not be able to convert to a ringtone. You will have to purchase it as ringtone at iTunes.
You can take a downloaded Mp3 import it into iTunes. Then with iTunes convert to ACC format adjust the play time to 30 seconds and use as a ringtone on the iPhone. The song does not have to be purchased as a ringtone or purchased from iTunes.
No trees were harmed by this message. However, several trillion electrons were extremely inconvenienced.