Having your own world to build in is essential to using the Creative server.
To purchase a creative world, use the command /creative buy. You will then be prompted to purchase a new world for 10,000 coins. Accepting will grant you a 256×256 area to build in. You may purchase as many worlds as you can afford.
When purchasing a world, you may specify the world type you would like by adding the modifiers flat or void to the end of the buy command. If neither is specified, your world will default to void.
Once you have your world, you can spend money to unlock additional features with /creative unlock. You can also expand your world with /creative grow, which grows the border by 256 blocks in all directions for 10,000 coins.