In the server console - the window where it is running, or the console in the hosting panel - the commands are written without a slash: op Steve, stop. The console is level 4, everything is available to it. The first option is given from there: in the chat /op can only be performed by a level 3 or higher operator.
The level that a new operator receives specifies op-permission-level in server.properties - default 4. The rest of the server settings are in section «server.properties», and how to raise a server from scratch - in the guide “How to create a server”.
Commands for yourself in a single player game - in Useful commands, how to open them in your world - in "How to include chats", all game commands with syntax are in the section "Comandes".