Launch the "iTunes" application on your computer. Select a song from the music library that you'd like to set as the ring tone on your iPhone.
Right-click on the song. Click "Get Info" and then "Options." You will see boxes labeled "Start Time" and "Stop Time." Input the start and stop times that you want to clip from the song into the corresponding boxes.
Right-click the song you've chosen and then click "Create AAC Version." This will create an abbreviated version of the song using the start and stop times you've specified.
Right-click on the AAC version you just created and click "Show in Windows Explorer." Navigate back to iTunes and click "Remove" to remove the AAC version of the song in iTunes. Click "Keep File" and save it to your computer's hard drive.
Click on the file and change its extension from ".m4a" to ".m4r." Click "Yes" when prompted to save the change. Double-click on the music file and save it back to iTunes.
Connect the iPhone to your computer using the USB cable and select "Sync" on the iTunes application to download the ring tone. Tap the "Settings" menu. Tap "Sounds" and then "Ringtones." Scroll through the list to locate the ring tone you just saved and tap on it to use it as your default ring tone.