[MysticalHippo]

You need an SD card. Put the SD in your phone then plug your phone into the USB port. Then go to Start>Computer>Removable Disk.(If AutoPlay pops up just click "Open folder to view files")
Once youre in there youll see all your phones file folders.
Copy the song you are using and paste it into the folder called "my_sounds". Then disconnect your phone.
In your phone go to Media Center>Music & Tones>My Sounds.
The song should be in the list. Touch your song and the "Options" screen should pop up. Select "Set As" then select "Ringtone".