Central Log In page

I am creating a log in page and need help with the form code. The overall design is already completed except for the log in area. I want a central page where my clients can log into the control panel, business class email or business class email pro accounts. You probably will have to have great knowledge of log in forms. The form must auto log in to the account. So instead of a customer going to particular domain example [login to view URL] and enter user name and password they can use the centralized log in page that I want developed.