Get the DNS name servers from the account information email you received from your web hosting company. In the rare case that you did not receive an account information email from the web hosting company, you should be able to find that information either through the admin control panel or by asking one of their customer reps.
Next, log in to your domain name registrar account and update the DNS (Domain Name Server) information. This will tell the system to assign a certain IP address or server location for your domain name.
This is the view from my Namecheap account.
Example DNS setup:
ns1.domain.com
ns2.domain.com
The domain name registrar will usually require you to input at least 2 DNS fields.