I’m new at coding anything.
In that case, you might want to consider starting with something easier than a LR plugin. 🙂 Seriously though, if you’re dead-set on making a LR plugin, I’d advise that you start out by copying exactly what your tutorial shows, (even if you have no use for the plugin). That will give you a little experience and confidence with making a basic plugin, and you can test it to make sure it actually works. After that, maybe you’ll be better equipped to branch out into unknown territory.
Can you have two copies of LR on one computer?
Probably not, but I don’t know!
Or is it sufficient to just have a second catalog?
Definitely use a “test” catalog, rather than your main one. And make lots of backups.