Please do not ask for complete solutions. We aren't in the business of doing your work for you. You need to try something, and when you run into problems, we will be more than glad to help you out. But don't just start a thread asking for code solving your problem.
MODERATOR
However, I can help you get started.
If this is a login/logout type of page, you can just use a database table to track when your users log in and log out. If not, you can catch every session started by using the global.asax file. The event is Session_Start. You can write an entry into a database table for every started session. But you are going to have a lot of work to do tracking the end of sessions. We've had a nice long
thread on this issue. I suggest you read it, try out the theories raised there, and come back to us if you need some help.