You can create a ringtone from a song using iTunes but you first have to get the song into iTunes to create your ringtone. This is done by opening iTunes, then going to the File menu at the top, selecting Add to Library from the drop-down list that opens, and locating the song on your computer. iTunes will then add (or "import") the song to your music library in iTunes. In the video when he say "go to" he's talking about selecting the song in your iTunes library that you want to create the ringtone from. You do this by clicking on Music on the upper left side of iTunes (see image below) and then finding on the song you want to use in the window to the right.
Once you locate the file he tells you to right-click your mouse on the name of the file and select "Get Info", then on the window that opens click on the Options heading and choose a start and stop time for your ringtone (must be 40 seconds or less). See if you can follow the video from there.
If you find this to be too challenging you may want to recruite a computer savvy friend to help you with this.