Teamspeak is an outside program that uses separate servers. You need to connect to a server (many larger fleets have them or Ventrillo servers - you'll need to get your server info from your fleet), and join a channel in the teamspeak window (servers are frequently set up to put new connections in an unused room, sometimes muted my default until they switch).
If you're referring to the built-in voice chat, almost nobody uses it. It's quality is poor, and silver accounts have to endure ads through it. So most people turn it off entirely.