First you will need a Website, so if you have not already set this up with one of the free ISPs (Freeserve, FreeUK, etc.) you sign up and get some web space and then grab an FTP client to upload your stuff.
Once that's up and running, then you can use on of the free to use ready made Message Boards, some of which are remotely hosted so that you don't have all the complicated clarts of setting it up. Try
http://www.cgispy.com/index.shtml
or
http://www.boards2go.com
or even
http://www.ezboard.com
although the last one has a much nicer paid for version that the free one and they try to get you to sign up for the paid version.
Anyway, whichever you choose, it's great fun and you will make loads of mates and learn lots once it's up and running. Remember, with the free stuff, the hosting people can pull the plug at any time without any conscience, and as you are not paying you have no rights.