To manage a server, you must be an "OP". To become an OP, please enter your player name on our web interface in the "Admin" area. (Edit game configuration -> Admin)

If the server is running in Advanced Mode, the server needs to be stopped and your player name needs to be entered in the ops.txt file.
One admin per line! (Edit game configuration -> ops.txt)

Now when the server is started, you will be an admin and you will be able to kick/ban players and give yourself items.
You will find a list of available commands here