There are videos and help sections on the net. I make my ringtones by editing a song or sound on audacity or sound booth what ever you have. A ringtone is a maximum of 30 seconds 29 to be sure. Save the file as an m4a. Which is mp4 audio file.
Find the file you just made on your computer. I would advise the desktop for now. Backspace the file extension and type in .m4r (this is the extension that tells iTunes to see file as ringtone)
Then add the files to iTunes.