Get into iTunes and you can do it for free on your own.
Go to the song and set the start and stop time (no more than 45 sec). Right click and select convert to AAC. Find the file in windows and right click it. Select rename and change the extension to m4r. Ignore the warning. Drag the file into ringtones and voila there it is. Don't forget to delete the original converted file from your iTunes library. On my webpage there is a more detailed set of instructions (www.tuaiw.com.