To stop search engines from indexing your site go to Settings > Reading and check Discourage search engines from indexing this site. This will discourage search engines from indexing your site (including your site in search results), but it is up to the search engines to honour this request.
Making individual posts private in WordPress
WordPress also comes with a built-in feature which allows you to make posts or pages private and password protected.
You need to start by creating a new post in WordPress or editing an existing one. On the post editor screen, you will see the Visibility option under the publish meta box. Click on the Edit link next to visibility.
There you will see the options to make a post Private or add a password protection to the post. Choose your desired option to make the post private or password protected.
If you choose to make a WordPress post password protected, then WordPress will ask you to enter a password. Users will need to enter this exact password to view the post. You can send this password to friends and family by email, and they can view the post by entering the password.
Important: Please note that users with the user role of Editor or Administrator can view a password protected or private post by logging into the WordPress admin area. However if you are the only user on your site, then you don’t need to worry about that.
To password protect your entire site you will need to use a plugin.