To restrict page access to certain pages, all you need to do is:
Click the Pages button on the left hand side of your screen
Find the page you're looking to change the access level of
Click the Gear Icon, then click the Show Settings button
Once you're in the page settings, you can then click the Page Access Dropdown and select your preferred access level.
You can have the page:
Public - visible to everyone
Password Protected - requiring a password to access
Members Only - only members of a specific group (of your choosing) would have access to that page