You either need a fax/modem on your computer (the one that you can plug a phone line to, similar to the ethernet port but smaller), or a fax server (usually in a corporate environment). If you are using a home computer with no modem, then I don't think you can use it. See
here on how to set up your modem (or fax server) for Windows Fax and
here on how to send the fax once you set it up.
There are also web sites which provide internet fax service. You can subscribe to one of them and send a fax through their web site. Some of them provide first month free.