I use .mp3 files for all my music in my iTunes library and to make a ring tone within iTunes is quite easy.
First listen to what ever file you want to use and work out the start and stop times for the section of music you want to use for your ringtone.
Right click on the music file and select "Get info" go to the options tab, fill the "Start Time" and "Stop Time" you want to use then close the "Get Info" window.
Right click on the music file and select "Create AAC Version" iTunes will use your preset start and stop times when converting the file.
Navigate to the folder where the file was created and change its file type from .mpa to .mpr and your ringtone is done. Click on it to open it with iTunes and it will be placed in your ringtone folder.
Don't forget to go back and remove the start and stop times from the original file when you are finished.
