When you register your domain name, your registrar will likely also supply DNS management, which allows you to point a domain at an IP address. Make sure your registrar provides DNS service! Enom.com, for instance, offers free DNS services. (so does network solutions, and presumably godaddy -- not sure about them though)
Once you're registered, simply set up an A record for the domain (and the www subdomain) that points to your IP. The A record is used by browsers (and other clients) when determining the IP to connect to when a specific host is requested.
And of course, before you start pointing things inbound, make sure you've got adequate firewall rules in place!