Turn Computer Into Phone How Can I Take An MP3 Song On My Computer And Turn It Into A Ring Tune For My Cell Phone?

How can I take an MP3 song on my computer and turn it into a ring tune for my cell phone? - turn computer into phone

I do my cell phone, a Sony Ericsson W580i pink cargo and play in the Walkman application, but do not let me select it as a ringtone.