The main ventrilo site can be found at
http://ventrilo.com/
You can find the download section from the link on the upper left hand of the site, or
http://ventrilo.com/download.php
You will want to download the Windows Client Platform version 2.3.0 - this should be the first download on the page.
Setup instructions can be found from the setup link on the upper left hand of the site, or
http://ventrilo.com/setup.php
Setup is simple. Once you have installed the Ventrilo client, run the program. In the interface, you will see three rows on top with dropdown menues titled User Name, Server, and Bindings.
Click the -> button next to User Name, click New, and enter what you wish your name to be. Finish by clicking OK. Although this can be anything you awnt, it is recomended that you use your character name.
Click the -> button next to Server, click New, and inter the name of the server. The server name can be anything you want, but it is recomended that you use something that will inform you what this particular server is for, such as the guild's name, DoA, temp, ect.
Once you have obtained the ventrilo information for the server you are using (usualy given during group formation for high end areas), you will enter the address or ip given in the 'Hostname or Ip' line, and the port number in the 'Port number' line. Unless given a password or default channel, ignore these. Click OK.
Ignore the Bindings row for now - it is not needed.
Click the Connect button on the upper right side of the Ventrilo client - if everything was intered in correctly, you will now connect to the server being used for whatever you are doing.
Now you should be ready to go - however it is recomended that you enter the setup section (setup button on the right side), and create a Push-to-Talk hotkey, as well as adjust the volume (Outbound and Inbound under Amplifiers - lower right hand setup section, voice tab).