You need two things. One, a viewer. For complete beginners, I recommend the Firestorm Viewer version 6.6.17. You can get it here (IMPORTANT: Download Firestorm for OpenSim, not for Second Life!):
https://wiki.firestormviewer.org/fs_older_downloads
Two, an avatar. Which also means a user account. You don't have it yet. This place is not the official OpenSim website where you register an avatar. It's only a third-party info site.
One important thing to know is that OpenSim is not a centralised, monolithic silo like Second Life or almost all other virtual worlds out there. It is not one world. It's many worlds, so-called grids. They all have different, independent owners, and they all run in different, independent places. OpenSim is akin to over 4,000 independent, big and small Second Lifes.
But almost all of them are connected to one another via the so-called Hypergrid. This means that you don't need an avatar on each grid where you want to visit a place. You can have an avatar on one grid and still visit most other grids with this self-same avatar.
I could tell you to go find a grid that suits you. But when I was a newbie, that wouldn't have helped me that much either. I know a thing or two about decentralised networks, and one of them is that the easiest way to on-board newbies is to tell them one specific instance to join.
The oldest grid in OpenSim and one of the two biggest grids is OSgrid. Go there to register an account and an avatar:
https://accounts.osgrid.org/?q=user/register
IMPORTANT: Names in OpenSim are always exactly 2 words, a first name and a last name.