how to display online user in side bar?

I got your first point. To display the no of users online you need to use Application state. Count the no of user's, set it in a session and access it in your master/web page. Check the links out:
A Walkthrough to Application State[^]
ASP.NET Application State Overview[^]
Application State in ASP.NET[^]

To display admin page you can set a link in master page. And ask user to enter admin UserID and Password. If the credentials provided by user is correct then redirect to Admin Page. Otherwise, you can use Asp.net Membership[^] to do that. See the links:
How To: Use Role Manager in ASP.NET[^]
Understanding ASP.NET Roles and Membership - A Beginner's Tutorial[^]



---Amit