Right-click on the tune and select "Create AAC version..." from the pop-up menu. Quicktime can also convert MP3 to AAC.
Ringtones need to be AAC (MPEG4 audio, aka .M4A) audio of less than 30 seconds in length. Once converted, find the file and change the suffix of the file name from .M4A to .M4R and drag the file into iTunes and it will be recognized as a ringtone.
There are a couple of applications that do this for Windows, and OS X provides a mechanism to do this with several tools and even provides means for batch conversion and automation of the process.