You can change the default gamemode of the server on the Files > Config Files > Server Settings page of your control panel. Simply change the "Game Mode" option to the setting you wish to use, save changes at the bottom of the page and restart the server.
Note that anybody who has already been on the server will not be automatically changed to the new gamemode - to do this you will also need to enable the "force-gamemode" option on the Server Settings page. Players that are opped are always exempt, but they can change their gamemode anytime with the "/gamemode" command.
You can also use the command "defaultgamemode <mode>" in the console, or "/defaultgamemode <mode>" ingame if you are opped, to change the default gamemode.