By "login system," I presume you mean you are making a website that allows users to login using a user ID and password. If you haven't already figured it out, this requires the use of sessions. Rather than storing passwords for your users in cleartext format, you should use PHP's password hashing functions to instead store a hash of the users's password generated with the password_hash function
Once you have your site using sessions and you figure out how to authenticate users and determine whether or not they are logged in, then you should be ready to work on CRUD.
It'd be difficult to explain how to create CRUD for your system without talking about things more specifically. What do you want to CRreate, Update, or Delete? This will depend on the database structures you define. The PDO documentation is full of information on how to perform insert queries (to create records), update queries (to update reords), and delete queries (to delete records).