Each appliance has it's own page which details how to log in. E.g. the LAMP page, scroll down to "Usage details & Logging in for Administration" and you can see that the user name to use is "root" and the password should have been set at firstboot. If you are launching from the Hub then you fill in these details during the launch of your appliance. For all other installs you set the password from within the appliance on first boot, although the method is slightly different for each build type.

If you tell me where you have your server running (e.g. from the Hub, in a VirtualBox VM, a Proxmox container, etc) then I can give you a more explicit answer.