Right click on the Server Name at the top of TeamSpeak and select the option "Edit Virtual Server".
Get More Options
Click on the "More" button to display the advanced features inside of TeamSpeak 3.
Changing the Host Message
Under the "Host Message" option you'll change the message next to "Message:". This is what will displayed when you first login to the TeamSpeak 3 server. Click "Apply" when your finished changing the message to have it reflect correctly.