You need to contact your registrar (the company you
registered your domain name with), and have them change the DNS
servers associated with your domain name (host records). Many
registrars let you do this through a web-interface.
Typically you would use names such as
"ns1.your-domain-name.com" and "ns2..." pointing to the IP addresses
of your own DNS servers.
Sometimes, it is necessary create these "host records"
(ns1... = IP address) before modifying you domain name
For example if you registered your domain name with
Network Solutions, you need to login to your account and use the
"Manage Host Servers" function.
If you registered through another
registrar, they may have a similar form or process this through
e-mail - contact them for their exact procedures.