Teamspeak has no connection with WoW, in itself. It is an independent program used to chat with people, whether in a game or not, sort of like Skype. Install the Teamspeak software, run the client program (not the server - I presume someone else is hosting the server) and in the "connection" menu, click the "connect" option to open the relevant page.
Next, you need the Server Name and the Server Address of the person hosting the Server, and also the password to let you in the chat channel. If your pal intends using the same server as you, you could note down this info from your PC, to simplify matters. Enter this info, then click the "connect" button, and that should be about all you need to do.
Edited, Jan 9th 2007 9:15am by Krago