I didn't know about Elder Scrolls Online until I joined the forums and saw it in the list. So how do I use the online version?

By the way, I just joined so I might take some time to understand where different types of questions are meant to be posted. Sorry if this doesn't belong here.

Elderscrolls Online (ESO) is an entire game itself. It is a MMO, currently only on PC, but is also coming to PS4 and Xbox One soon.
You can find out lots of info at the official site. http://www.elderscrollsonline.com/en-us/agegate

The ESO forum is hosted on its own site.