Cheats

If you're looking to use creative mode / other admin commands you'll need to enable permissions first.

If you're on a server:

In your server console type op username where username is your minecraft username.

If you're on singleplayer:

Go to (Pause Menu -> Open to Lan -> Cheats ENABLED -> Done)

You'll have to do this every time you close and open your world on singleplayer.

You can now type /gamemode creative in chat to go into creative.

You can find a full list of commands on the Minecraft wiki.