Without installing extra software, any AAC format file can be made into a ringtone, if you have it in MP3 format, use iTunes to create an AAC version of the song (File > Convert > Create AAC Version).
Once it is in AAC format, the file extension will be .m4a, copy the m4a file to the desktop or another folder outside of the iTunes library folders. Then, change the file extension to .m4r using the Finder > Get Info.
Finally, import it back into iTunes. Since it has .m4r extension, iTunes understands it to be a ringtone and categorizes it as such. Now, you can sync it to your phone.
Make sure you use the Song Info > Options in iTunes to specify start and stop times for the ringtone. Typically you want a file of no more than 40 seconds.