Each account comes with a free Basic world, which supports a small number of players and maps. To create your world, got to your profile page and there will be a message at the top. Enter a world name and click the create button. World names can contain only letters, numbers, and spaces. If the world already exists, you will be given a message and prompted to choose another. Otherwise your world will be created and you can begin building.
If you already have a Basic world, it will be listed in your worlds with note that you are the owner.