If you notice any errors on the site you can login and propose 'edits' to the site these are then subject to votes by other users of the site to ensure that the changes proposed are accurate. If you are not confident to propose an edit yourself than you can describe the problem in the forums and someone may be able to help you.

There is no direct method of registering to edit the site, the logins are done using openID through existing logins, most people will already have an openID (though may not be aware of it). If you don't have an openID then you can sign up for one at myOpenID.