Hey Coal,
Unfortunately, if you want a plain
www._______.com email address, you will have to pay to register the domain.
These usually charge per year (those charging per month are probably offering web hosting as well, which you do not need)
I personally used google apps for my last domain (though I believe they register it through a 3rd party) and it was around $10 for a year.
If you do not urgently need to claim a specific address, I would advise you use something free, like
http://dyn.com/dns/ while you are just playing around and learning.
There is no real need to have the .com address until you are going live to the public.
The advantage of DynDns is that it will automatically detect your changes in IP (depends on your ISP how often your IP will change), and the address you end up with is not too obscure.