There is no direct way to set a song as a ringtone.
If you are on mac and have garageband, you can drag your song into it, use the loop bar at the top to set a segment to 40 seconds or less, then go to share, send ringtone to itunes.
If you're on windows I'm not really sure what you would do but there are solutions.
You can also buy iToner to make ringtones, but where's the fun in that