More seriously, having a login form on a website means there’s an authentication process, which means with a very basic example:

1. Create a database
2. Create an backend application interacting with the database
3. Create a record type in your database representing your users
4. Make a website that interacts with the previously created backend application
5. Create a sign up page that ask username/password and send them to your backend application which will check its unicity and save it into the database.
6. Create a login page that ask previously created credentials, send them to the backend application in order to check that they correspond to a user record in your database, and retrieve a specific authentication key that uniquely identify the user.
7. Create pages that need a valid authentication key.