Is there a way to secure only one part of a wordpress site? I mean not the entire website. I have not found any solution yet. Thank you.
While there are a few different options to locking down WordPress, one simple method of locking down a post or page would be to use the Protected post option within the page editor.
If you are more interested in locking down some of the more well-known security issues of WordPress, I recommend viewing this presentation: Access Denied.