To change your gamemode on a minecraft server use the /gamemode command.
This is how it is used:
/gamemode <mode> [player]
Example:
/gamemode creative aft2d
The available gamemodes are: survival, creative, adventure, spectator. What the different gamemodes are for and what they do exactly is explained here.
The name of the player is only required if you want to change the gamemode for another player or if you use that command from the console.
You have to be OP!
Please note that you have to be OP in order to use that command. This article shows on how to get OP.
If you use a permission system make sure that you have the appropriate permissions assigned to your group/player
Gamemode Options on the server page:
You are probably wondering what the gamemode options on the options page are for.
First of all there are the two setting options "Gamemode" and "Force Gamemode."
The option Gamemode specifies the default gamemode for new players who have never played on the server before.
If Force Gamemode is activated, even players who have already played on the server will be set to the default gamemode when entering the server.