This is only a 3rd party info website. OpenSim is not a monolithic, all-made-and-run-by-one-corporation world.
First you need to create an avatar. But you can't do it here. In order to make this easy, go here:
https://accounts.osgrid.org/?q=user/register
Your avatar will need a forename and a surname.
Next, you need a viewer. A client. An app to install on your computer so you can use OpenSim. The most popular one is Firestorm. You can get it here:
https://wiki.firestormviewer.org/fs_older_downloads
Download the version for OpenSimulator and for your operating system. Do not download the Second Life version!
Next, install Firestorm and launch it. At the bottom, you'll see various fields: one for your full avatar name, one for your password, one that doesn't matter yet, and below, one for the grid (= world) that your avatar is registered on. That last one, set it to OSgrid.