Problems managing users in System Preferences

I'd like to remove admin rights from the main user and have admin rights for a separate user as this is good practice. But, even after I press the "click the lock to make changes" button" I cannot remove admin rights nor delete any users. The checkbox for admin is dimmed and so is the delete user button. Currently I have two users both with admin rights and want either to remove admin rights from one or delete one and start again if for some reason I can't edit them. I read somewhere that you can't remove admin rights from your original user set up when you first got the mac but I've created another user with admin rights so I can't see why this should be. There are, according to VO, two buttons both labelled "click the lock to make changes" - does it matter which one I press? Can anyone guide me how to delete one of the current users (they both have admin rights) and/or remove admin rights from the latest one I created?