How to change a Minecraft server into creative mode? I'm using serverminer?

Usually you just do /gamemode c followed by the username of the player, for example /gamemode c Notch do the same but using s for survival and a for adventure.

To change everyone to creative, do /gamemode c @a

That way the server would still be survival but certain users will be in creative mode, to make the world creative by default you need to edit your server.properties and change the gamemode to 1 instead of 0