Are you giving them your site (domain, hosting, forum) or just having someone run your forum?
If you're just letting somene run your forum you'd just make them an admin.
If you're giving someone your site you'd have to transfer your domain name to them; they'd do this at any domain registry online, initiating a domain name transfer, and you'd have to accept the transfer request. Then they'd have to either get hosting or take over billing at your current host, and then they'd have to transfer files if they moved hosts.
