If you have a godaddy account and need to create your own name servers (i.e. ns1.mydomain.com and ns2.mydomain.com) here is how it is done:
1- Login to your domain registrar control panel (Go Daddy for example) by clicking on "My Account"
2 - after you login and from the "Domain Names" drop down menu, click on manage domains
3 - locate the domain for which you would like to create nameservers
4 - click on your domain name
5 - Click on "Domain host summary"
6 - Click on "click here to see details or to modify"
7 - Enter ns1 in the Host field as your first name server and name server IP as provided to you in your welcome e-mail
8 - Click on "add new host" and do the same for ns2 then "save changes"
9 - allow from 24 -72 hours for the changes to take effect (normally much quicker, 10-15 minutes)
10 - Now you can point your domain to your custom created name server from your control panel
The process is very similar with any registrar on the web.