If you're sitting in front of a pool-computer you will see the login-manager first. You have to use the login-manager to log in the computer.
You can also set the environment after logging in, i.e. you can select the window manager or the session. This is possible via the menu item “Session”.
If you are over the quota the graphical login is not or only with errors possible. In this case you can either change to the local console using the keyboard shortcut “STRG + ALT + F2” (to return to the graphical mode use “STRG + ALT + F8”) or you use ssh to Connect to the virtuell Pool computer. You should immediately begin to clean up. you can find tipps under Quota.
On this screenshot different window manager are listet. The standard selection here is Cinnamon. If you wand to change this create a file with the name .xsessionrc and enter the path to the required window manager there. For i3, for example, this is done as follows:
user@client36 ~ $ echo "exec /usr/bin/i3" > .xsessionrc
TODO test