What is the official web site to register or park a domain name?
There is no official site as such (though networksolutions would like you to think they are...).
There are dozens of popular places. Your best bet is probably your existing ISP or, if you intend a high(ish)-bandwidth web site, the company that will provide your hosting package will probably throw in domain registration either as part of the package or for a few dollars extra per year.
Be careful of the very low cost ones as some charge you $20 or so if you want to transfer the domain away from their servers at a later date. I can't recommend any US based ones as I use UK ones (www.clara.net & www.freevirtualservers.com) - not that you HAVE to use a US one if you are in the US, it just makes payment easier.
It's worth doing some research so you know what your options are (for example, do you want .com, .net, .info or .us domain(s)?
domain registrars (Maybe Sean could move this thread to The Lounge)
There are hundreds of ICANN-sanctioned registrars, many or most of which offer a "parking" service. Parking isn't necessary unless you've already told someone about the domain and want to have some kind of content there.
I can highly recommend easydns.com as both a registrar and a DNS provider.
I would urge you to stay as far away from networksolutions.com as you can. Hideous customer service and responsiveness. Once upon a time, they were the only game in town. Hasn't been that way for quite a while, but they still act like it.
Most registrars have help information and a list of frequently-asked questions and answers, but heavily weighted toward using their service. Maybe if someone knows of a good neutral tutorial on domain registration and website setup, they could chime in.
My wife owns and operates our web business www.flatheadvalley.com and we have always recommended www.godaddy.com to our customers, we have found them very helpful, reasonable on pricing and never had them hold a domain hostage, which allot of the other registars do, I would agree, stay away from network solutions, they have been the big bully in the domain name world for a long time now.