How to authorize a client on the site?

Question

Java

I have a site with two buttons on the main page: log in and register for authorization and registration, respectively. When you try to register, first the password and the second password are checked, then the email is checked, if this is already in the database, then this user is already registered, if everything is fine, then the data is placed in the database. Next, we need to log in, and here I no longer understand how to do it. I can check the email and password for the presence of such a user in the database, but I don’t understand how to display his personal account on the site later. That is, before authorization, the personal account should be empty, and after all actions should be displayed for a specific user.