Hi, I’m new to SS. I know you need to login to access cms, but how can I apply this to the whole website. Like when a guest visit a url, this will be redirected to the login page and come back after login.
Actually, I’m reading others source code, but I really couldn’t find how this is achieved.
Thanks.
This is a standard part of the CMS. If you look at the ‘Settings’ tab on a page, you can choose who is allowed to view the page:
If you set this to ‘Logged in users’ then the system will automatically redirect any non-logged-in users to the login page, and then return them afterwards. By default, sub-pages will inherit these permissions from their parent.
If you need something a bit more, you can also do this in code by making a couple of changes in the default Page Controller class (see tha docs page here: https://docs.silverstripe.org/en/5/developer_guides/controllers/access_control/#controller-level-checks)
Thank You! Everything becomes easy now! 
