For doing this you have to change Post Visibility Settings
This can be done when you are posting a post.
just see on right side or near publish button
there will be a visibility option which is public by default this can be edited..
Visibility: Public
just click edit and choose password protected radio botton and give a password of your choice
That all.
But wait if you don’t want it to be password protected then change the Permalink Setting in setting
and create a custom URL or your choice..
That’a all