Registering a domain does not actually acomplish much... The assumption would be that you actually what to USE (host) that domain name. So registration is only the first part.
Most of the companies listed above will do everything, but it is valuable for you to first have an idea of what "everything" is.
The chain of events to get "everything" going is somthing like this:
1) Register/secure the domain
2) Decide where this name will point to... i.e. a hosting provider or your always-on home computer.
3) Find out the static IP address of the machine that will be yourdomain.com
4) figure out a way to get DNS Hosting... that's the thing that will poing yourdomain.com to the IP address in step 3
5) Now that your domain name is pointing to the right palce... what are you going to host there? A web site? An FTP site? Your own email SMTP server?
I've broken this all out because most registrars or hosting facilities will offer to do all of these individual things for you... each with a small cost.
If you do anything, make sure you don't let your hosting facility register the domain name in THEIR companies name... If you do that you'll have a big battle if/when you want to move to another hosting company.
...I hope I did not just confuse you more...
