After successfull connection to the server you will see the main game window. It is divided to four parts initially, but we will get familiar with each of them in the next topics.
If your client isn't configured to automatically login you to the game you will be asked to supply your username and password to join the game (or observe the game).
The login window is located on the top left of main screen:
There are two types of logins on most servers - guest user and authenticated user.
If you type "guest" as your name or push Ctrl-g you will not be required to supply password. There are some differences between guest users and authenticated users. The major difference is that no information is written to the server players database for guests. So when you disconnect from the game all your statistics are wiped out.
For authenticated users everything is saved to players database, so when you come next time to the server and supply your name and password you will start the game with statistics that were saved for you from the last session.