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.
You can also use GarageBand to make ringtones. It is a standard app that comes with all macs. I haven't done it in awhile but I remember it being very easy to learn to do. In a nutshell, I chose the desired song from my iTunes library, and used GarageBand to snip it to the exact part of the song I wanted to use and create an m4r file. Once you are done with this, simply put your new file in the folder with the rest of your ringtones, then sync your phone to get the new ringtone on it.