All items for your website have an Access Level setting. In the example below you can see the Access Level column in the Article Manager page. This column exists for menus, modules etc as well. You can restrict access to items by simply clicking on the green text in the 'Access Level' column, next to the item you wish to change. In the example below you would click on the word 'Public' next to an item. When you click on the word, it will change from 'Public' to 'Registered'. If you click it yet again, it will change from 'Registered' to 'Special'.
If you wish to hide an item so that it is only visible for registered members who log into your website, set it to 'Registered'.

When you set access for an article to 'Registered' and someone tries to view it when they're not logged in, they'll get a message that they have to log in to view this item. You can also set access from within an article rather than from the Article Manager, eg

When you set access for a menu item to 'Registered' and someone is not logged into the site, they will not even see the menu item. It will only appear after you've logged in.