For the Portal Engine one, it's pretty simple, go to the Pages application and at the root of the site, click on the General > Security tab and select the radio button to require authentication. This will force the whole site to require a login. You'll need to create user accounts to allow the users to log in.
For the MVC site, you start out similar in forcing the site to require authentication, however, you'll need to implement the login page and authentication within your MVC application. I'd suggest checking out the DancingGoat example site for this code.