First, there's not sextiong of the panel for creating and editing your site. You'll have to do that one on your own. If you're only going to have a forum and nothing else, then you don't need to worry about any other site design.
If you want to get into some site design I'd reccomend you check out NVU for a WYSIWYG (what you see is what you get) editor. And Filezilla for a FTP client. You'd use NVU to design your site, and then you'd use filezilla to transfer your files form your comptuer to DH.
If you're wanting ot check out your site before the DNS propigates from your domain name, you can set up a 'mirror'. Just add a new domain from the panel and set it up to be a mirror. create whatever.dreamhosters.com! It's that simeple and now you can check out your site. Of course once your DNS propigates your own www.domain.com will work as well.
Did you see the one click install for phpBB forum in the panel? You might like just using that. It's very easy to get set up.