When creating the server, you must enter at least the domain name and a description. If multiple server templates are defined on the system, you can select one from the menu - templates determine various defaults for the new server, such as the website configuration, DNS records and so on.
To select which features are enabled for the server, use the checkboxes in the second part of the Advanced options section. Sub-domains have a much smaller set of available features compared to sub-servers or top-level servers.
At the bottom of the Advanced options section you can select to have all web traffic to this new site forwarded to another URL. You can also select to have the new virtual server use its own private IP addresses, which is required for SSL and anonymous FTP hosting.