If you would like to use the RCON to do any server wide commands you cant do ingame, download a free software called mcron which will help you do this, this is especially useful for initially enabling the OP permission for yourself the easy way.
Download mcron HERE, extract that zip file into a folder on your computer and double click on the "launch" icon,
A cmd window will open up where you can log in, your RCON Details are found on your server Control Panel page.
In the command window, type your server IP address and press enter, then type your server port number and enter,
lastly, type in your RCON password and press enter, and then you can start issuing any commands to the server.
To give a player OP permission:
op [player name]