This shit is too confusing for me. I have no idea what benz, Kiwi and Tim are even talking about, it's going right over my head.
registering a domain name means you now own the name, nothing more.
then you have to rent space on someone's computer to host the website
then you have to upload the files that people are going to look at when then visit that website.
you want to have a forum where people can post and read messages.
so you need to
1) register the name at someplace like godaddy. $10/year
2) purchase hosting from someone, $50-100/year. you probably need to find a hosting company that supports cpanel, php and mysql.
3) pay someone to install the software you want, such as the software that getbig uses, SMF
and probably
4) pay someone to manage the site for you
if you want to take the time to learn, you can do 3 and 4 yourself