Newby - Adding a web site "post it" pop-up window.

Newby - Adding a web site "post it" pop-up window.

I need help. Where do I start? I want to add a help/reminder/post-it style pop-up window While I am, searching for, or while I am on a web site. Lets call it a "helper" window. Not to a site I am programming, but to any site on the web.