You need two separate DNS servers (a primary and secondary) to host a domain. Both must have static, public IP addresses. For that reason many people don't do their own DNS, but use something like zoneedit, which is a free DNS service. Also, DNS can be easy to screw up if you don't know what you're doing, so you might want to look into that route.
But basically once you have set up your name servers, you just tell the registrar you got the domain name from (most have a form) and off you go.