Can anyone tell me what would be the best way to design a database that would allow for users to register on the website but the member details will also carry over to a forum so they don't have to register again.
Yeah, using something like phpBB would be a better solution that writing one from scratch. You could then use phpBB's user tables as your own, ie your user login form would actually read and write to the phpBB tables, so if someone logs in on your site they are also logged into the forum.