Registering a Domain on the internet?

I graduated college in Computer Systems and networking. But didn’t learn anything about domain names on the internet. I have a consumer running a windows200 server on DSL with IIS installed and a email server installed. He wants to be able to type in a web address and get to his windows200 server machine and have a personal email address (username@domain of his chose.com). What kind of prep work do I need to do before I register a domain? How do I go about doing it? He has a permanant ip address.