I think you can change the game mode by adding the -gamemode [gamemode name] argument to the same .exe that you add -dedicated to, but I might be remembering this incorrectly. This will set the gamemode to what you want. Or, join the server, press CTRL + A, click server settings (I think) and choose a different gamemode.
Server messages can be done by typing messageAll('', 'hi'); in the console (note: there are two 's). There are ways to stylize this text. I've also seen people using announce("hello"); but I'm not sure if it's default.