and I heard about openGL being superior to normal
Sure, if you can deal with a lack of certain visual effects, any colormap-based effect being completely incorrect, some surfaces being flat-out invisible, the renderer still being nearly as CPU-bound as software... I mean, proper perspective is definitely a perk, and it's still mostly usable.
But this isn't a place for me to soapbox about renderers. If you wanna use the OpenGL renderer, run whichever EXE you prefer running the game with using the -opengl parameter. The full command will look something like this:
srb2win.exe -opengl
(If you aren't sure how to do this, if you open SRB2's folder in the file explorer, make sure you don't have any files selected, and right-click an empty space while holding shift, there should be a context menu option that says "Open command window here". Then you type that in there. Alternatively, type it into a Notepad file and save it in your SRB2 folder as "opengl.bat" or whatever you'd like to call it; the .bat means it acts as a command, or technically a series of commands, to run in a command prompt.)
Do keep in mind that the renderer is still missing a lot of features, and isn't officially supported by most of the dev team (over soft, anyway), so some things might not work properly.