This documentation best describes the process of
DNS and
nameserver propagation.
DNS propagation refers to the time that it takes between pointing your
domain name to a new server, and the rest of the internet seeing that change.
To put it into more practical terms, when you register your domain it will not be immediately accessible by you or anybody else on the internet. This is because the new domain has not propagated to the rest of the internet yet. This process can take anywhere from 1 to 48 hours.
Unfortunately there is no way to speed up the DNS propagation process, it is something that takes time.