This is done on the Teamspeak 3 client when you are connected to your server:

1. Right-click on the name of your server.
2. Click 'Create Channel'.
3. Name the channel.
4. Add other information as necessary (like a password if you'd like it to be an officer's forum, etc.)
5. Select 'Permanent' under 'Channel Type'.
6. Click 'OK'.

You're done!