Well, you have to get rid of everything else on the main site, and ONLY use wordpress when you do that. Basically, you just move all the files in /wordpress/ up a directory, and then change the install directory in the wordpress config. You can, technically, still use other folders inside the main site, but wordpress can screw up access to those sometimes, so I wouldn't really recommend it, unless you really know what you're doing. For now, you may just want to make your main page a redirect to /wordpress/