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 settings.
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.
Trouble logging in? Simply enter your email address OR username in order to reset your password.
For faster and more reliable delivery, add firstname.lastname@example.org to your trusted senders list in your email software.