If you decide to move your site to a VPS or a dedicated server, then when you try to place your own nameservers for your domain, you will receive the message "You must enter a registered nameserver" and will not accept them, turning the whole thing into nightmare.
Before you start, you must have two IPs, which is provided by the company where you transferred your site. This guide is based on Godaddy, but there will be no tremendous differences in other providers.
From the Control Panel, go to Domains > Manage My Domains and from there to the domain you want, click "View/modify". From there, go down to Host Names and click on "Manage". In the new window, select "Add one now" and type "NS1" as the hostname and in the IP Address you enter the IP corresponding to ns1.domain.com.
Do the same for ns1.domain.com, where you put NS1 as the hostname and in the IP Address you put the corresponding IP.
When you finish, you will be able to add the nameservers you've just entered to your domain name "Nameservers". Remember that DNS changes may take up to 48 hours to appear. You can check the status of DNS here and here.