To use rcon from outside the game, such as to kick a player so that you can get into your server, you can either use the "Server control" page in our control panel, or a 3rd-party client that supports the Quake4/Doom3/ET:QW rcon protocol.
To use rcon from inside the game, pull down your client's console with the ~ button and enter these commands to start off:
Replace "serverip" with your hostname here (identifier.g.nfoservers.com, for instance), and replace "rconpassword" with the password defined in your server.cfg file by "net_serverRemoteConsolePassword".
After doing this, you can enter various commands to the server. For instance,
rcon admin kick playernum - Kicks a user with a certain index number (from 0-15) from the server; the order appears to be based on when players joined
rcon admin kickAllBots - Kicks all AI bots from the server
rcon admin restartMap - Restarts the current map
rcon spawnServer mp/aquarium - Starts up a particular map