Sadly to many, I am passionate about caravanning and the freedom it creates. I would like to create a forum where caravanners can comment on the sites that they have visited so that others can benefit from their experience.

Can anyone give me some ideas on where to start?

Why "sadly"? Sounds like a worthwhile interest to me.
I'd start by visiting existing sites to see if there's a need. That would be a good way to get in touch with fellow enthusiasts to find out what they might want.
You could also consider an email list, which could do the same thing, but much more simply.
For a forum, all you need is a web host, a domain name and some forum software like:
http://www.phpbb.com/
See here for more software:
http://en.wikipedia.org/wiki/Compari...forum_software