You should ask your webhost how to set this up.
Unless you've set up your own domain name servers/service you probably need to set the ns1 and ns2 servers, in the domain name's control panel, to the domain name servers (DNS) provided by your host.
From there it will be up to your webhost to explain what to do. They probably have tutorials, faqs or help forums that explain how to set up DNS, redirections, multiple domains, etc on their servers/system.
There are many versions of server config, control panels, etc. That is why questions such as your's are usually best answered by the host.