Edit name servers in the domain management area. If you're buying domain and web hosting from a single provider, like Hostgator, then the domain name automatically has a corresponding host.
But if you buy a domain (from name.com) & host (hostgator) from different providers, then you have to replace the server names in domain (host.com) with the server names of Hostgator in order for your site to work..
* Login to the domain registration site on which you've registered your domain
* Once you are signed in, search for your domain and click on it
* You can see your default name server. Click on the "edit name servers" option
* Remove the default name server one by one
* Enter the new name server that is the server name of your host provider and click on add