If your have your domain and nameservers that need to be set, this is done through your registrar. You should be able to set your nameservers on their website.
Go to your control panel on your domain registrar then find where to enter your name servers, then do so. It normally takes 24 hours to update with most registrars
You can give your informations to the server administrators and they will update your settings for you. It's the best way if you feel uncomfortable doing it, and you're sure you will not change settings by error.