You can host it on your computer, which will most likely be slow, depending on how good your computer is. With ventrilo you can only host an 8 person server yourself, if you need more than an 8 person server you need to buy it. Setting up a server can be difficult if you are not very computer savvy, but their are many tutorials, including a good one on ventrilo's website. I recommend visiting ventrilo's set-up page and looking at the Server installation, not the client installation. http://www.ventrilo.com/setup.php
Please remember if you do decide to host your own ventrilo server, it will only run when your computer is on, and you have the server running.