You can download the Ventrilo client at http://www.ventrilo.com
The information for the guild's server is in the guild message of the day, that includes the IP, port and password for the voice server.
You can use anything for the username, but it would probably be best to use one of your character names, just so everyone knows who is who.
There's also a Ventrilo overlay add on that will display the speaker's name in the game when they are talking. I haven't tested this yet, but if you want to try it you can download it here:
See you and hear you online!