I have a problem, i want to know how to create login activity in a website which many people can connect at the same time

Answer 1

You should learn PHP and SQL.
1) Make a connection to the database with PHP.
2) Check if the username exists in the database.
3) Check if the password matches in the database. If the username and the password matches let the user in by setting PHP-SESSION-variables. If it doesn't, redirect the user to the login page and add an error message.

Answer 2

I think you are talking about using a server, right ? Try to see PHP with HTML, it probably have what you need